1. A collision prediction device mounted on a own vehicle, comprising: an image sensor unit that extracts an object on an image and detects a size and a position of an object on the image; a millimeter-wave sensor unit that transmits a millimeter wave and detects a distance between the own vehicle and an object present in a sensing range; a sensor matching unit that determines that the object detected by the image sensor unit is the same as the object detected by the millimeter-wave sensor when a change rate of a size of the object detected by the image sensor unit on the image is in disagreement with a change rate of a distance from the own vehicle to the object detected by the millimeter-wave sensor, and when the objects are determined as identical, defines the object as a prospective collision obstacle, and calculates a relative position of the collision obstacle and the own vehicle after a predetermined time, based on the position detected by the image sensor unit and the distance detected by the. millimeter-wave sensor unit; and a collision prediction unit that predicts whether there is going to be a collision between the own vehicle and the collision obstacle from the position calculated by the sensor matching unit.