To instantaneously and continuously estimate the coefficient of friction of a road surface and judge the road surface by calculating the characteristic quantity according to the moisture content of an object to be measured from the detection signal of the reflected wave from the road surface, and comparing it with a reference area value.

First and second light receivers 1, 2 receive first and second infrared rays of the light projected by a projector 3, irregularly reflected on the road respectively, and convert them into electric signals. An amplifier 7 amplifies the output first reflection signal of the first light receiver 1, and a low-pass filter 8 removes the disturbance noise component therefrom followed by outputting. Comparators 9, 10 that received this input compare it with thresholds A1, A2 showing high and low levels of the road surface water, and output the resulting comparison data corresponding to the road surface water to a CPU 15. The output second reflection signal of the second light receiver 2 is also inputted to comparators 13, 14 in the same manner and compared with thresholds B1, B2 showing high and low levels of the road surface diffused light, and the resulting comparison data corresponding to the diffusion degree of road surface is outputted to the CPU 15. The CPU 15 estimates the coefficient of sliding friction of the road surface by calculation followed by outputting.