A shaft and rolling bearings are mounted in the housing. A magnetic sensor is fixed to a first end surface of a shaft. A circuit board carrying a magnetic sensor is fixed to a surface of a lid of the housing at the second end. As a calibration voltage feed line, a cable is connected to the circuit board and extends outside through a hole formed through the lid. With the lid fixed in position, thereby applying a preload to rolling bearings, calibration voltage is supplied to the magnetic sensor through the calibration voltage feed line, thereby writing initial calibration parameters into the magnetic sensor from an external calibration device. After writing the parameters, the calibration voltage feed line is connected, at the outside, to a stabilizing output end of a power source input/output board fixed to the surface of the lid at the first end.