A sensor (20) arranged in a deep groove ball bearing (10) includes an encoder (22) attached at an inner ring (1) to be rotatable integrally with the inner ring (1), and a magnetism detecting element (24) attached at an outer ring (2) via a sensor housing (26) to face the encoder (22) with a sensor gap. Then, the magnetism detecting element (24) is capable of measuring a rotating state of the encoder (22) rotating in the rotating state identical to that of the inner ring (1) in conjunction with a rotation of the inner ring (1). The deep groove ball bearing (10) includes a foreign material entering prevention portion that prevents a foreign material from entering the inside of the bearing from a gap between the inner ring (1) and the sensor housing (26). The size of the gap between the inner ring (1) and the sensor housing (26) is substantially identical to the sensor gap between the magnetism detecting element (24) and the encoder (22), and the gap configures non-contact seal and functions as the foreign material entering prevention portion.