PURPOSE: To perform tests under the same conditions as actual running conditions of a vehicle, by differentiating the difference between the speed of a body and the rotating speed of a following wheel, and identifying the hydroplaning state of the following wheel when the differentiated value is a specified value or more.

CONSTITUTION: A driving-wheel rotary-speed sensor 10 and right and left following-wheel rotary-speed sensors 11 and 12 output pulse numbers corresponding to the rotary speeds. A CPU 5a in a counting and analyzing device 5 computes the body speed V0 of a vehicle 3 and the rotary speed V of the following wheel and records the speeds in a recorder 15. The difference between the body speed V0 and the rotary speed V of the following wheel is obtained and stored in a RAM 5c. Whether the difference exceeds a specified value or not is judged. The time, the speed difference and the like are outputted to a printer 17.