Original Japanese cuisines are not spicy but Japanese love hot spicy food.
Spicy food is considered as a good way for metabolism. If you eat a spicy food, you will be sweating soon, even you are sitting in an air conditioned room. On the other hand, you will never sweat if you are eating fish and chips in an air conditioned room. The spicy food make your body hot and your sweat is trying to cool your body down. Then, your body will try to get rid of unnecessary stuffs from your body during this procedure.
Major spicy foods in Japan are curry with rice and ramen noodles. Although normal ramen noodles are not spicy at all, there are many ramen bars which are specialized in spicy noodles and they do not offer any normal ramen noodles.
If you are looking for a spicy food in Fukuoka city, please visit Houten Honke. They are opened till 3AM! The restaurant is full of people who enjoyed drinking at Nakasu district.
However, watch out. If you are not used to eat spicy food usually, you will become a flamethrower!