3 a and 3 b rotation speed changing actuators 4 a and 4 b independently changing engine rotation speeds n a and n b of the pair of left and right engines 3 a and 3 b a pair of left and right outdrive devices 10 a and 10 b respectively connected to the pair of left and right engines 3 a and 3 b and rotating screw propellers 15 a and 15 b so as to propel a hull 2 forward reverse switching clutches 16 a and 16 b disposed between the engines 3 a and 3 b and the screw propellers 15 a and 15 b a pair of left and right hydraulic steering actuators 17 a and 17 b respectively independently rotating the pair of left and right outdrive devices 10 a and 10 b laterally electromagnetic valves 17 aa and 17 ba controlling hydraulic pressure in the hydraulic steering actuators 17 a and 17 b a joystick 21 accelerator levers 22 a and 22 b and an operation wheel 23 as operation means setting a traveling direction of the ship an operation amount detection sensor 39 see fig5 as an operation amount detection means detecting an operation amount of the joystick 21 operation amount detection sensor 43 a and 43 b see fig5 as operation amount detection means detecting operation amounts of the accelerator levers 22 a and 22 b an operation amount detection sensor 44 see fig5 as an operation amount detection means detecting an operation amount of the operation wheel 23 and a control device 31 see fig5 controlling the rotation speed changing actuators 4 a and 4 b the forward reverse.