PURPOSE: To obtain an optical detection sensor possible for a low power consumption, by flickering a light emitting diode and decreasing the illumination time in comparison with the non-illumination time.

CONSTITUTION: When an optical path between a light emitting diode 1 and a photo transistor (TR) 3 is switched as shown in (a) because of the mechanical displacement of a grating 5, since a TR7 is switched with an output of an oscillation circuit 6, the light emitting diode 1 is flickered in the period T as shown in (b). Then a light shown in (c) reaches the TR3 and it is inputted to a monostable multivibrator 9 as an electric signal. When the output of the multivibrator 9 has a pulse width as shown in (d) which is longer than the period T of the diode 1, the output is as shown in (e). That is, a signal similar to the (a) representing the mechanical displacement of the grating 5 is obtained.