Yes, this is a spider! It looks like an ant, but if you look closely you’ll see that it has eight legs and lots of eyes. The theory is that they use their disguise either to help them sneak up on ants to eat them, or to help them avoid being eaten by predators themselves.
While I was watching one male ant spider, another male dashed up and tried to take over his territory. They had a quick battle, just like a pair of stag beetles (or even stags!) and the original one managed to push the intruder down the tree. After a few centimeters of being pushed, the intruder gave up and ran away. A true miniature battle!
Web page showing the different species of ant spiders in Japan: here.
Location: Near Genkai town office (Google map)