A steering wheel adapter comprises a base having an opening, a plate with a plug formed, and a ring capable of sliding and rotating along the outer face of the base. In the steering wheel adapter, a guide hole penetrating the side face of the base from the outer face into the opening is disposed, a locking member capable of protruding into the opening is provided in the guide hole, and a cam face is provided facing the guide hole on the inner face of the ring. When the ring is slid and rotated in a state where the plate is mounted on the base to insert the plug into the opening, the cam face pushes and protrudes the locking member into the opening, and the locking member is pressed to contact the outer surface of the plug. Consequently, both easy attachment/detachment and prevention of rattle can be achieved.