I thought I was really taking a photo of an ant on a leaf… but after looking at the preview of the first shot to see whether it was in focus, I suddenly realized that it was in fact an ant-mimicking spider (arigumo)! Amazing mimicry. It was only around 5mm so I couldn’t see much detail with my own eyes (poor eyesight!) but looking at the photos, you can see that it is holding up its front legs to look like antennae. It seems that this kind of jumping spider might mimic ants in order to hunt them, or maybe as a protective measure to prevent themselves from being eaten by predators (birds or wasps) that typically avoid ants because of the formic acid they produce.
Family: Salticidae (jumping spider)
Myrmarachne spp. (Myrmarachne japonica?) = arigumo