A rotation-speed sensor device is provided to comprise an encoder supported by a rotating race of a rolling bearing unit for vehicle wheel support to be concentric with the rotating race and having a detected portion the characteristics of which are alternately changed in a circumferential direction, a rotation detection sensor supported by a non-rotating member on the vehicle chassis side and having a detecting portion facing the detected portion to produce a detection signal, a wireless transmission device for sending the detection signal from the rotation detection sensor to a controller provided on the vehicle chassis side, and a self-diagnosis circuit to judge whether there is an error or not in the rotation detection sensor.