When it is determined by an acceleration condition determiner that an own vehicle is accelerating, a collision prediction time calculator calculates a collision prediction time in accordance with a relative speed and a relative distance, and a timing setter sets an activation timing to a reference timing. When it is determined by the acceleration condition determiner that the own vehicle is not accelerating, and by a preceding object determiner that an object is not a preceding object, the collision prediction time calculator calculates the collision prediction time in accordance with the relative speed, a relative acceleration, and the relative distance, and the timing setter is configured to set the activation timing to be larger than the reference timing.