A video game device calculates an angle of an object 71, etc., with respect to a reference direction in a three-dimensional space. Then, the video game device defines a virtual cylindrical model for each of a plurality of objects so that a bottom surface of each cylindrical model is perpendicular to the reference direction. The video game device defines a plurality of cylindrical models for a predetermined object if the inclination of the predetermined object with respect to the reference direction is greater than a predetermined angle. Then, the video game device determines whether or not the defined cylindrical models have an overlap therebetween. Then, if two cylindrical models are determined to be overlapping with each other, the video game device gives a predetermined change to the two objects in response to the collision.