It's a museum dedicated to Matsumoto Seicho - a prolific crime fiction writer. If you're not familiar with him, this might not be such a worthwhile trip, especially if you don't know Japanese. The museum hands out english pamphlets on request, but the exhibit is in Japanese. There is also a film viewing room, where one screening is about an hour and a half.
There is also a library in the basement if you need to do research on him.
Bought the ticket as part of the Kokura castle/gardens entrance package.
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