An encoder 12 changing a characteristic thereof alternately and at equal intervals with respect to circumferential direction is supported by and fixed to the hub 4 concentrically. A detecting portion of a sensor 13 supported by the outer ring 3 is closely opposed to a detected face of the encoder 12. Width dimensions of first, second detecting portions provided at the detected face are changed continuously with respect to direction of operating a load to be detected. In accordance with a change of the load, a pattern of changing an output signal of the sensor 13 is changed. Thus, the load is calculated by observing the pattern. By performing filtering process on a signal expressing the change in the pattern, a variation based on the mounting error or the like is removed.