PURPOSE: To reduce sailing resistance acting on a submerged part, by forming the submerged part of a casing to smaller size further more flattened type.

CONSTITUTION: A forward gear 18 or a reverse driving gear 19 integrally operated respectively with a driven shaft 14 at forward or reverse sailing is to receive both turning effects directly applied from a driving gear 16 and applied through the reverse gear 19 or the forward gear 18 and an intermediate gear 30 from the driving gear 16. Accordingly, the driving gear 16 provides the turning effects in two routes while the gears 18, 19 receive the turning effects from the two routes, and a surface pressure generated in each meshing point between each gear 16, 18, 19, 30 is decreased to an almost half of the pressure in conventional gears, then each gear 16, 18, 19, 30 can be formed to small size, in this way, the submerged part of a casing 11 can be formed to smaller size further to a more flattened type.