PURPOSE: To realize flow-rate measurement under no influence of environmental condition, by making light into pulses through the mechanical motion of a flow meter side corresponding to a flow rate and transmitting the pulses through an optical fiber.

CONSTITUTION: When a shaft 13 rotates at a rotating speed proportional to the passing flow rate of the flow meter, gears 15 and 16 also rotate at speeds proportional to the passing flow rate, and the integral value of the passing flow rate is displayed on a display 25. A cam 18, on the other hand, rotates together with the gear 16 and the reversion force of a leaf spring 20 is impressed as a force for rotating the cam 18 fast regardless of the motion of the gear 16. Therefore, the cam 18 rotates fast and waits for a projection 19 to arrive at a stable position. Light incident from a light emitting element 5 to the optical fiber 3 through a reflecting mirror 22 is reflected and then travels backward to enter a photodetector 6. Its output is passed through a differentiation circuit 22 and a monostable multivibrator 33 to display the integral flow rate on a counter 34. Thus, the flow-rate measurement under no influence of environment condition is carried out.