PURPOSE: To enable the highly reliable detection of an amount of flexure, by projecting light beams from a base part toward the front end part thereof.

CONSTITUTION: When a light beam 20a falls on a point Ps of a light receiver 30, an electromotive force proportional to the energy of the beam is generated, and a an electric current is delivered to electrodes 30a and 30b in the direction of the Y axis and to electrodes 30c and 30d in the direction of the X axis. The Y- direction coordinate of the incident point Ps can be obtained by the computation of the difference in current between the electrodes 30a and 30b by a difference amplifier 40, and the X-direction coordinate of said point by the computation of the difference in current between the electrodes 30c and 30d by a difference amplifier 50. When the position of the light-receiving surface of the light receiver 30 is changed by the flexure of an arm 10, signals corresponding to the amounts of divergence in the directions X and Y of the position of the origin of the light receiver 30 are delivered from the amplifiers 40 and 50, and the amount of the lexure is detected therefrom.