An ECU acquires relative position information related to a relative position between a host vehicle and another vehicle (A, B, C) traveling ahead of the host vehicle, and error information related to an error in the relative position, detects the position of a preceding vehicle (A) traveling in front of the host vehicle, identifies the position of the preceding vehicle (A) by comparing the relative position based on the acquired relative position information with the detected position, and if relative position information oh a plurality of other vehicles (A, B, C) is acquired, identifies the position of the preceding vehicle (A) by comparing the relative position based on the acquired relative position information with the detected position, by using a threshold that is varied in accordance with the error information.