To prevent a foreign matter from entering a case storing a drive mechanism.

An opening/closing means 22 allows to open only the through area of a connection part 20 between a carrier member 13 and a female screw body 16 (driven member) of the opening part 12 of the case 11, and allows to displace the opening area according to the movement of the connection part 20. When the connection part 20 moves the opening part 12 according to the movement of the carrier member 13, the area of the opening part 12 not penetrated with by the connection part 20 is closed by the opening/closing means 22. In the area penetrated with the connection part 20, also the opening part 12 is almost closed by the presence of the connection part 20. Thus, there is no possibility of the external foreign matter entering the case 11 through the opening part 12.