To optimize a clutch connection control for clutches (15, 16) of a forward/reverse rotation switching mechanism (17). A reduction reverse gear (9) includes a hydraulic clutch-type forward/reverse rotation switching mechanism (17) that converts rotation power of a main engine (7) mounted on a hull (2) to an output of forward rotation, neutral, or reverse rotation and transfers the output to a propeller (5), a forward/reverse rotation switching valve (24) that controls operation of the forward/reverse rotation switching mechanism (17), and a trolling device (36) that controls pressure of working oil into the forward/reverse rotation switching mechanism (17). When the forward/reverse rotation switching valve (24) is switched to a forward (F) or reverse rotation position (R) during normal propulsion, the trolling device (36) controls a pattern that reduces pressure of working oil into the forward/reverse rotation switching mechanism (17) so that the pattern is different between when the forward/reverse rotation switching valve (24) is in the forward rotation position (F) and when it is in the reverse rotation position (R).