In a rotation angle sensor attaching structure for detecting rotation of a rotational angle detection object shaft through a reduction gear, an outer race of a bearing is force fitted in and secured to a central portion of the reduction gear, and an inner race of the bearing is secured to a supporting wall by a supporting bolt. A sensor-connecting element of a rotation angle sensor, which extends across a head portion of the supporting bolt, is secured integrally to an outer side face of the reduction gear. Such arrangement achieves a minimal size of the rotation angle sensor attaching structure.