PURPOSE: To improve resistance to noise and provide an excellent insulating characteristic and separability from other system by detecting load with light as a medium by making use of the characteristic of an optical fiber which increases a light transmission loss with a side pressure.

CONSTITUTION: Light Lis is made incident into an optical fiber 1a on a load detection side from a light transmitter 2. The light Lo generating a light transmission loss by passing through a load detecting part 5 is transmitted to a light receiver 3a on the load detection side. The same light Lis as 1a is made incident into an optical fiber 1b on a reference light side from the transmitter 2 and the reference light Li generating no loss with load is transmitted to a light receiver 3b on the reference light side having the same characteristic as the receiver 3a. An arithmetic circuit 4 receives the signals Lo', Li' amplified and converted by the receivers 3a, 3b and calculates the load from the light transmission loss by operating log (Lo'/Li'). The part 5 transmits the load to a body 7 for applying a nonuniform side pressure upon receiving load W and applies the nonuniform side pressure to the fiber 1a.