To prevent simultaneous connection of a first speed clutch and a second speed clutch so as to prevent seizure of each clutch without making both a first speed solenoid valve and a second speed solenoid valve by connecting in series the valves.

A first speed side solenoid valve 3 and a second speed side solenoid valve are connected in series to a dc power supply 1. At the time of generation of wire breakage in wiring on the way or generation of failure in any one of them 3, 2, 4, a valve element 25 of the valve 3 opens a pipe 6 similarly with the case that a switch 2 is put OFF while a valve element 26 of the valve 4 is closed, a first speed clutch 8 is connected while a second speed clutch 9 is disconnected, and sailing of a ship can be made at the first speed. With a failsafe mechanism capable of running at one speed, the ship can sail in this way without drifting. Through simultaneous operation of the valves 3, 4, generation of trouble can be eliminated.