A motor-fastening mechanism fastens an output shaft of a motor and an input shaft of a driven member to each other. The motor-fastening mechanism has a fastening section which fastens the output shaft of the motor and the input shaft of the driven member relatively immovably, a motor-attachment member which is attached to the motor, an engaged section which is provided on either one of the motor-attachment member and the driven member, and an engaging section which is provided on the other of the motor-attachment member and the driven member and which is engaged to the engaged section such that displacement of the output shaft in a rotating direction thereof is regulated.