A vehicle speed V is compared with a threshold Vth; when V is less than Vth, TLC is set to an initial value t0; when V is not less than Vth, TLC is set shorter with increase in V. Then TLC is compared with a predetermined value t th, and when TLC is less than t th, TLC is replaced by t th being a lower limit. This improves compatibility between a reduction in an occurrence frequency of needless warnings and a guarantee of a deviation margin time.