PURPOSE: To reduce current consumption very much in a light emission and reception mechanism arranged with a small gap in an almost enclosed housing, by setting a forward current, which is supplied to a light emitting diode, at 1.0mA or less.

CONSTITUTION: In a digital micrometer, a light emitting diode is secured near one side of an index scale and a rotary scale. A light receiver consisting of a light detection diode is secured near the other side of said scales. When a forward current IF is 1.0μA, the output current ISC of the light receiver is about 1.5μA and the SN ratio is about 21 dB. According to this constitution, a transmission pulse which can be sufficiently signal-processed can be detected even if the forward current of the light emitting diode 32 is set at 1.0mA or less. This results in reducing current consumption.