PURPOSE: To measure a speed at a high accuracy with a simple construction by a method wherein light transmitting through or reflected on a moving object to be measured and outputted differentially being introduced to separate slit trains.

CONSTITUTION: Light from a light source 21 is irradiated on an object 22 to be measured. The light transmitting through the object 22 is focused with a condenser lens 11 and introduced to a hundle optical fiber 12 which leads the light to a beam splitter 14 through a condenser lens 13. The light transmitting through the beam splitter 14 moves to a first slit train 15A while that reflected thereon moves to a second slit train 15B. The intensity of the lights transmitting through the slit trains 15A and 15B is introduced as output to light-electricity converters 17A and 17B separately through optical fibers 16A and 16B thereby enabling the control of a speed at a high accuracy with a simple construction.