A collision determination device for determining a collision occurrence against a door part of a vehicle has: a door acceleration detector that detects an acceleration generated on the door part of the vehicle and a direction of the acceleration; and a collision determination part that determines that a collision has occurred against the door part from the outside, when the direction of the acceleration detected by the door acceleration detector is a closing direction of the door part, and determines that no collision has occurred against the door part from the outside, when the direction of the acceleration detected by the door acceleration detector is an opening direction of the door part.