A four-row tapered roller bearing is incorporated in the clearance between the inner surface of a housing and the outer surface of a rolling roller. A pair of supporting members supporting sensor devices are fitted in and supported by a part of the inner surface of the housing in such a manner that they are opposed to the ends of outer rings of the four-row tapered roller bearing disposed close to the both axial ends thereof. The sensor devices each comprise a distortion gauge as a detecting portion and a first coil for transmitting as a wireless signal a signal obtained by processing an output signal outputted from the distortion gauge. A second coil for receiving the wireless signal transmitted by the first coil is retained on a part of a member other than the supporting members.