A control unit 5 calculates, by a statistical processing, the current front/rear-directional acceleration, the current front/rear-directional velocity, and the current front/rear-directional position of the three-dimensional object to be determined while considering an error caused by a camera. These values are used in the statistical processing so as to calculate the front/rear-directional acceleration after Δt seconds, the front/rear-directional velocity after Δt seconds, and the front/rear-directional position after Δt seconds, and so as to obtain a probability of contact after Δt seconds based on the front/rear-directional position after Δt seconds.