PURPOSE: To know the ground speed of a moving object by forming a secondary power source on the ground surface in front of the moving object, and detecting the reflected waves which do not abide by the rule of reflection from that sound source in a ground speed meter utilizing ultrasonic waves.

CONSTITUTION: A transceiver is provided to a moving object. The ultrasonic wave signal (e) generated in a transmission signal generating circuit 12 is projected from a transmitter T to the front ground surface. The reflected wave thereof is received in a receiver R and is amplified by a high-frequency amplifier 1. The output of the amplifier 1 is mixed in a mixer 2, by which the frequency of the difference between the respective frequencies fR and fL of the reflected wave and a local oscillator is obtained. The output of the mixer enters an analog multiplexer 3, from which the output corresponding to the timing in which the reflected wave exists is taken out and is applied to a BPF5. If the moving object is moving at a preset speed, an output is produced, and an electronic buzzer 7 is driven, thereby announcing that it is a normal speed. With such constitution, the ground speed of the moving object is known.