To precisely judge the frozen state of a road surface even when a antifreezing agent is scattered by setting a temperature sensor for detecting the road surface temperature and a wet sensor for detecting the wet state of the road surface, and measuring the change with time of the measured values to judge the freezing of the road surface.

This method comprises a road surface temperature sensor 8 for measuring the temperature of a road surface and an infrared reflectance measuring type wet sensor 9 for detecting the wet state thereof. An arithmetic output device 10 measures these measurement data in a fixed time period, and integrally processes the measurement data to judge and output the presence or absence of freezing. Namely, in the arithmetic output device 10, the road surface data measured at intervals of several minutes are stored on a memory, the temperature value in a certain point of time is taken as the base to compare it with the difference with the temperature value in the next point, and the temperature change width between the both is detected. When the temperature change width is within the measurement error range of the temperature sensor 8, it is judged constant, and when the constant temperature difference and time are prescribed values or more, a freezing is judged.