A gear mounting portion of a rotating shaft includes a shaft portion, with a D-shaped cross-section, engaged with a connecting portion of a gear having a D-shaped cross-section; the connection portion provided with a first hole portion; one flexible portion provided along the axial direction; and a retaining portion provided at a tip of the flexible portion. The retaining portion protrudes outwards with respect to the shaft portion, and the shaft portion is engaged with the connecting portion after the retaining portion has passed through a second hole portion of a hollow shaft. The flexible portion remains bent while the retaining portion passes through the second hole portion of the hollow shaft. Once the shaft portion engages with the first hole portion of the connecting portion, the flexible portion ceases to bend and engages with the gear. As a result, without damaging the rotating shaft, the gear is reliably prevented from coming free from the rotating shaft.