PURPOSE: To improve detecting accuracy by determining the speed of a vehicle with respect to the ground based on the time from the transmission of a transmitted wave to the reception of the wave with a receiving part after the reflection from a road surface in addition to the relationship of the frequencies of the transmitted wave and the received wave.

CONSTITUTION: Both a transmitting part 12 and a receiving part 14 are fixed at points close to the front-wheel attaching part of the bottom surface of a body so as to face the forward direction in the slant attitude for the road surface. The transmitting part 12 is connected to a ground-vehicle-speed determining means 24 through a pulse driving circuit 20. The receiving part 14 is connected to the ground-vehicle-speed determining means 24 through a receiving amplifier 22. The means 24 is constituted with a computer as a main body. The means 24 transmits an ultrasonic wave at the constant frequency for about every 5ms at an interval of about 3ms. The reflected wave from a road surface is received with the receiving part 14. The distance to the road surface is computed based on the delay time from the transmission of the ultrasonic wave of one pulse to the reception with the receiving part 14 after the reflection. Thereafter, the fluctuated amount of the distance from a reference value and the fluctuated amount of the slant angle with respect to the road surface are computed. The ground vehicle speed is computed based on these values, the reference value of the slant angle and the received frequency.