A movement determining map is stored in addition to a field display map. The field display map is a three-dimensional map for displaying a field of the video game on the image display screen. The movement determining map is a two-dimensional map in which an enterable region. An unenterable region is distinguishably provided for the characters existing in the field. A control section determines a movement route of the sub character, in which no unenterable region exists, using the movement determining map when a predetermined movement route determining condition is met. The sub character is caused to move in accordance with the determined movement route. The predetermined movement route determining condition is met every predetermined period of time or in the case where the main character moves apart from the sub character in a predetermined distance or more.