A collision determination area set for a dog as an object in a virtual three-dimensional space is, for example, a sphere having the center at the chest of the dog and having a radius of 30 in the “on all fours” state, a sphere having the center at the hip of the dog and having a radius of 10 in the “standing” state, and a sphere having the center at the head of the dog and having a radius of 10 in the state of the dog “trying to fawn with another dog” in the virtual three-dimensional space. Using the collision determination area set in this manner, it is determined whether or not the dog has collided against another object. Therefore, a collision determination suitable to individual situations can be realized while suppressing an increase in the amount of calculations required for the collision determination.