A ship propulsion unit (4) is fixed to a turning cylinder (8) which is turnable relative to a ship body. A pinion (12) of a drive shaft of a servo motor (3) is engaged with a turning ring (10) of the turning cylinder (8). The servo motor (3) is connected to a servo amplifier (2). A controller (1) and an operating wheel (16) on a bridge side and a control board (18) and a manual turn button (17) in a steering engine room are connected to the servo amplifier (2) via a changeover switch (19). A tracking transmitter (14) provided on the turning ring (10) is connected to the controller (1). A turning position command inputted from the operating wheel (16) causes the servo motor (3) to be driven. The actual turning position of the ship propulsion unit (4) is detected by the tracking transmitter (14), and servo control is performed to bring the deviation between the turning position specified by the turning position command and the actual turning position to 0. It is possible to switch control to the steering engine room and carry out manual control. Accurate turning position control can be realized for a turnable ship propulsion unit.