A gear case part (70), which has an upper aperture that opens upward and which forms a recess that houses a portion of a drive gear device (63) and for which the drive gear device (63) can be inserted from the upper aperture and assembled, is provided integrally on the upper part of a first or second divided case (21, 22); a cover (66) that blocks the upper aperture of the gear case part (70) is fastened to the gear case part (70); an operating part (67) protrudes upward from a through-hole (66a) that vertically penetrates the cover (66); and by removing the cover (66) from the gear case part (70), the drive gear device (63) can be extracted from the gear case part (70) through the upper aperture.