To attain a supporting plate holding structure which can ensure strength against external force from a gear without making it bigger nor increasing the number of parts used.

In a motor unit 10, a supporting plate 66 which supports the upper ends of the shaft parts 36, 48 of the first gear 34 and the second gear 44 and the lower end shaft part 82 of the final gear 78 is fitted into a retaining part 100 which is formed at a case body 14. The end surfaces 116, 118 of a corner part 68 are retained under such a condition as they are brought into contact with longitudinal wall parts 102, 104 of the retaining part 100. When external force from the first gear 34 which rotates or the like works upon the supporting plate 66, stress therefrom works upon the respective end surfaces 116, 118 of the supporting plate 66 and the longitudinal wall parts 102, 104 of the retaining part 100 as compressive stress. Therefore, the contacting area of the end surfaces 116, 118 and the longitudinal wall parts 102, 104 which can bear the stress are ensured sufficiently in the case body 14, thus ensuring sufficient intensity without increasing its size.