A computer of an image processing device executing an image processing program displays, on a display device, a virtual three-dimensional space where there are a plurality of objects. The computer varies a size of a collision detection region defined for a predetermined object so that the size repeatedly increases and decreases over time, while the predetermined object is moving. Then, based on the collision detection region, it is determined whether or not the predetermined object and another object are in contact with each other. The computer displays, on the display device, a predetermined effect being inflicted on the object determined to be in contact with the predetermined object.