Species within the genus Eurema (Grass Yellows) range from Asia, Africa, Australia, and Oceania, to the Americas.
The Grass Yellows are generally weak flyers, fluttering low around bushes and flowers.
At least 71 species are currently known. 
They are generally difficult to tell apart and what makes it even more difficult is that the sexes often differ slightly and there are different seasonal forms (wet and dry season forms).
The main differences between the species are:
a) the number of black spots on the underside of the forewing, combined with
b) the shape of the (apical) black markings on the upperside of the forewing and
c) the shape of the apical area markings on the underside of the forewing.  

There are 5 species within the genus Eurema in Japan: Eurema blanda, Eurema brigitta, Eurema hecabe, Eurema mandarina and Eurema laeta.
The butterfly on the photo hardly shows any spots on the underside of its forewing making it Eurema laeta, sometimes also refered to as the Spotless Grass Yellow.

Kihoku area, Mie Prefecture, Japan, 28 September 2018