A counter is provided to count an output pulse which a pulse generator generates each time a vehicle moves by a predetermined distance. A microprocessor is provided to actuate a timer in synchronism with the leading edges of the output pulse from the pulse generator. The timer is set to a constant time interval during which the speed of the vehicle is to be detected. The microprocessor calculates the speed of the vehicle from the count of the counter which count is a value during the time interval from when the timer starts to operate to when the pulse generator generates a pulse just before or after the end point of the set time interval. Thus, the speed of the vehicle can be detected with good resolution and precision.