To solve the problem that the resolution power of an opening signal of a gate is deteriorated because the overall stroke is decelerated to one rotation of a rotational shaft to which a potentiometer is attached to perform the detection, as the variation in a resistance value.

A pulse detector is attached to a driving shaft of the gate, and the stroke is detected as the pulse signal. After determining the normal rotation or the reverse rotation by the pulse signal, each signal, as the count value, is outputted to the primary arithmetic part and a secondary arithmetic part. The primary arithmetic part performs the arithmetic processing, and performs the full-scale display by the output to a full-scale display means. The output of the secondary arithmetic part is outputted to a partial display means for performing the enlarged display of only a part of the overall stroke.