diff --git a/app/Exceptions/CsvImportException.php b/app/Exceptions/CsvImportException.php
new file mode 100644
index 0000000..eee24cf
--- /dev/null
+++ b/app/Exceptions/CsvImportException.php
@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Exceptions;
+
+use Throwable;
+
+class CsvImportException extends \RuntimeException
+{
+    /** @var string[] */
+    private $errors;
+
+    /**
+     * CsvImportException constructor.
+     * @param string[] $errors
+     */
+    public function __construct(array $errors)
+    {
+        parent::__construct(array_first($errors));
+        $this->errors = $errors;
+    }
+
+    /**
+     * @return string[]
+     */
+    public function getErrors(): array
+    {
+        return $this->errors;
+    }
+}
diff --git a/app/Io/CheckinCsvImporter.php b/app/Io/CheckinCsvImporter.php
new file mode 100644
index 0000000..b02d309
--- /dev/null
+++ b/app/Io/CheckinCsvImporter.php
@@ -0,0 +1,76 @@
+<?php
+
+namespace App\Io;
+
+use App\Ejaculation;
+use App\Exceptions\CsvImportException;
+use Carbon\Carbon;
+use Illuminate\Support\Facades\DB;
+use League\Csv\Reader;
+
+class CheckinCsvImporter
+{
+    /** @var string CSV filename */
+    private $filename;
+
+    public function __construct(string $filename)
+    {
+        $this->filename = $filename;
+    }
+
+    public function execute()
+    {
+        // Guess charset
+        $head = file_get_contents($this->filename, false, null, 0, 1024);
+        $charset = mb_detect_encoding($head, ['ASCII', 'UTF-8', 'SJIS-win'], true);
+        if (array_search($charset, ['UTF-8', 'SJIS-win'], true) === false) {
+            throw new CsvImportException(['文字コード判定に失敗しました。UTF-8 (BOM無し) または Shift_JIS をお使いください。']);
+        }
+
+        // Open CSV
+        $csv = Reader::createFromPath($this->filename, 'r');
+        $csv->setHeaderOffset(0);
+        if ($charset === 'SJIS-win') {
+            $csv->addStreamFilter('convert.mbstring.encoding.SJIS-win:UTF-8');
+        }
+
+        // Import
+        DB::transaction(function () use ($csv) {
+            $errors = [];
+
+            if (!in_array('日時', $csv->getHeader(), true)) {
+                $errors[] = '日時列は必須です。';
+                throw new CsvImportException($errors);
+            }
+
+            foreach ($csv->getRecords() as $offset => $record) {
+                $ejaculation = new Ejaculation();
+
+                $checkinAt = $record['日時'] ?? null;
+                if (empty($checkinAt)) {
+                    $errors[] = "{$offset} 行 : 日時列は必須です。";
+                    continue;
+                }
+                if (preg_match('/\A20\d{2}[-/](1[0-2]|0?\d)[-/](0?\d|[1-2]\d|3[01]) (0?\d|1\d|2[0-4]):(0?\d|[1-5]\d)(?P<second>:(0?\d|[1-5]\d))?\z/', $checkinAt, $checkinAtMatches) !== 1) {
+                    $errors[] = "{$offset} 行 : 日時列の書式が正しくありません。";
+                    continue;
+                }
+                if (empty($checkinAtMatches['second'])) {
+                    $checkinAt .= ':00';
+                }
+                $checkinAt = str_replace('/', '-', $checkinAt);
+                try {
+                    $ejaculation->ejaculated_date = Carbon::createFromFormat('Y-m-d H:i:s', $checkinAt);
+                } catch (\InvalidArgumentException $e) {
+                    $errors[] = "{$offset} 行 : 日時列に不正な値が入力されています。";
+                }
+
+                $ejaculation->save();
+            }
+
+            if (!empty($errors)) {
+                throw new CsvImportException($errors);
+            }
+        });
+    }
+}
diff --git a/tests/Unit/Io/CheckinCsvImporterTest.php b/tests/Unit/Io/CheckinCsvImporterTest.php
new file mode 100644
index 0000000..a85935d
--- /dev/null
+++ b/tests/Unit/Io/CheckinCsvImporterTest.php
@@ -0,0 +1,37 @@
+<?php
+
+namespace Tests\Unit\Io;
+
+use App\Exceptions\CsvImportException;
+use App\Io\CheckinCsvImporter;
+use Tests\TestCase;
+
+class CheckinCsvImporterTest extends TestCase
+{
+    public function testIncompatibleCharsetEUCJP()
+    {
+        $this->expectException(CsvImportException::class);
+        $this->expectExceptionMessage('文字コード判定に失敗しました。UTF-8 (BOM無し) または Shift_JIS をお使いください。');
+
+        $importer = new CheckinCsvImporter(__DIR__ . '/../../fixture/Csv/incompatible-charset.eucjp.csv');
+        $importer->execute();
+    }
+
+    public function testMissingTimeUTF8()
+    {
+        $this->expectException(CsvImportException::class);
+        $this->expectExceptionMessage('日時列は必須です。');
+
+        $importer = new CheckinCsvImporter(__DIR__ . '/../../fixture/Csv/missing-time.utf8.csv');
+        $importer->execute();
+    }
+
+    public function testMissingTimeSJIS()
+    {
+        $this->expectException(CsvImportException::class);
+        $this->expectExceptionMessage('日時列は必須です。');
+
+        $importer = new CheckinCsvImporter(__DIR__ . '/../../fixture/Csv/missing-time.sjis.csv');
+        $importer->execute();
+    }
+}
diff --git a/tests/fixture/Csv/incompatible-charset.eucjp.csv b/tests/fixture/Csv/incompatible-charset.eucjp.csv
new file mode 100644
index 0000000..866ed4d
--- /dev/null
+++ b/tests/fixture/Csv/incompatible-charset.eucjp.csv
@@ -0,0 +1,2 @@
+����,����,���������
+2019-01-01 00:01:02,�ƥ��ȥƥ��Ȥ���������,https://example.com/
diff --git a/tests/fixture/Csv/missing-time.sjis.csv b/tests/fixture/Csv/missing-time.sjis.csv
new file mode 100644
index 0000000..99b9d73
--- /dev/null
+++ b/tests/fixture/Csv/missing-time.sjis.csv
@@ -0,0 +1 @@
+�m�[�g,�I�J�Y�����N,�^�O1
diff --git a/tests/fixture/Csv/missing-time.utf8.csv b/tests/fixture/Csv/missing-time.utf8.csv
new file mode 100644
index 0000000..b0bc641
--- /dev/null
+++ b/tests/fixture/Csv/missing-time.utf8.csv
@@ -0,0 +1 @@
+ノート,オカズリンク,タグ1