PURPOSE: To make the action of a mobile element recordable free from effect of noise due to electromagnetic induction by detecting the action thereof having various color elements arranged with a prism and a photodiode.

CONSTITUTION: Color elements red, yellow, green, blue and violet in color are arranged on a mobile element 5. Light from a light source 1 is focused with a lens 4 and irradiated on the color elements passing through a light guide 3. The light reflected by the color elements enters a prism 8 and is focused by colors passing through the lens 4, a light guide 6 and a lens 7. The light is detected with photodetectors 10 arranged by colors. According to the movement of the mobile element 5, the output of the photodetectors 10 varies thereby enabling the detection of the movement thereof 5.