PURPOSE: To detect the extent of extension or contraction adequately over a long period of time by using the information on the extension and contraction directions from optical displacement detectors that detect extension and contraction movements and a control valve.

CONSTITUTION: A light emitting device 16 and a photodetector 17 are provided to the tube 6a of an oil hydraulic cylinder for driving a boom. A light shielding member 19 provided with notches 18 for passage of light at every other suitable interval in the longitudinal direction is stuck at one end to the rod 6b of the cylinder and is supported at the other end by a guide 20 provided to the tube. In accordance with the extension or contraction of the cylinder, the light from the light emitting element to the photodetector is interrupted and pulse signals are emitted. The extension or contraction direction is detected with the oil hydraulic control valve of the cylinder and in accordance with said direction, a reversible counter is changed over, and if the above-mentioned pulses are counted with the counter, the extent of the extension or contraction of the cylinder is discriminated.