To achieve a precise calibration by acquiring a gradient, where an output signal changes as brake-related amount, as an actual inclination and calibrating a brake-related-amount sensor.

A brake operation force sensor 60 detects, as a brake-related- amount sensor, the axial force of an input rod as a physical amount corresponding to a brake operation force Fp, and outputs a continuously changing signal. A computer 110 calculates a voltage value V from the electrical signal of the brake operation force sensor 60, substitutes the voltage value V in a conversion function f(V) of a RAM 116, and calculates the brake operation force Fp. In this case, the voltage valve V when a stop lamp switch 40 is off (a brake pedal is not being operated) is set to V1. Then, a voltage value when a brake operation force switch 50 changes from off (the brake operation force is equal to or less than a set value) to on (a set value or higher) is set to a voltage value V2. An actual gradient is acquired based on the voltage values V1 and V2 and the conversion function f(V) is corrected, thus calibrating the brake operation force sensor 60.