A PCS ECU 20 in a PCS system 10 calculates TTC which is a brake operation timing of a host vehicle, on the basis of the relative velocity Vr between the host vehicle and an object, a predetermined deceleration amount ΔV caused by a brake operation of the host vehicle, and a deceleration a caused by the brake operation of the host vehicle. Thereby, the deceleration amount ΔV caused by the brake operation of the host vehicle is to be a set constant amount regardless of the relative velocity Vr between the host vehicle and the object. Accordingly, it is possible to ensure the deceleration amount ΔV caused by a more appropriate brake operation.