PURPOSE: To make it possible to mount a transmitting/receiving part or an antenna on the bonnet of a vehicle and to attain operation with sufficient mobility by measuring a speed only when the transmitting/receiving part or antenna or the speed measuring instrument is moved within a prescribed range or completely stopped.

CONSTITUTION: A rotation detector 13 is connected to a transmission 12 of the vehicle mounting the speed measuring instrument. The movement of the vehicle is detected by an output signal from the detector 13, and when the moving speed exceeds a previously determined value, a reset signal is outputted to stop the speed measurement so that the speed can be measured only when the vehicle is stopped or moved only within the previously determined value. Consequently, whether the movement of the vehicle is affected to the speed measurement or not can be decided and the speed can be measured only in the case of no influence, so that the speed measuring instrument with sufficient mobility can be obtained.