fantia image downloader - 「fantia」 大量イメージダウンローダ
This is a PHP script that can download all posts in a fanclub on fantia.jp that you are authorized to access. It does not allow you to circumvent any paywalls.
I developed it with PHP 7.2, but it should work with later PHP 5.x versions aswell. It needs the cURL extension, that should be it.
php dumpia.php --key <key> --output <path> [--fanclub <id>] [--verbose] [--downloadExisting] [--exitOnFreePlan]
You will need to supply a key as an argument. The key is the value of the _session_id cookie in your browser when you are logged into the site. You can find it using various methods, likely most accessible to you is your browser's developer console.
The fanclub parameter takes the fanclub ID you can find in the URL when viewing the fanclub gallery.