A rolling bearing includes an outer stationary ring, an inner rotating ring, and a plurality of rollers disposed between the inner and outer rings. An optical sensor is mounted on the outer ring. A computer receives the output of the optical sensor to calculate the revolving speed of the rollers from the number of rollers that pass the sensor per unit time, as detected by the optical sensor. The computer then calculates the time-integrated difference between the revolving speed of the rollers and the rotational speed of a rotary shaft on which the inner ring is mounted, as detected by a rotational sensor. Based on the thus obtained difference, it is possible to detect skidding of the rollers at an earlier stage.