A turn control system for a ship propulsion unit which turns a ship propulsion unit installed in a ship rotatably about a turning shaft to a desired position, the turn control system comprising: a first operating unit for inputting turning position setting information specifying a position to which a ship propulsion unit is to be turned; a first control unit for outputting a turn command signal specifying, based on the turning position setting information inputted from the first operating unit, a direction in which the ship propulsion unit is to be turned and an angle by which the ship propulsion unit is to be turned; a servo amplifier for outputting a speed command signal based on the turn command signal from the first control unit; a servo motor which is engaged with a turning ring provided in the ship propulsion unit rotatably about the turning shaft; and a tracking transmitter for detecting a turning position of the ship propulsion unit and outputting information on the detected turning position as turning position detection information; wherein control is performed in the first control unit such that the turning position detection information from the tracking transmitter and the turning position setting information inputted from the first operating unit is compared and such that, when there is no deviation between the compared information, a turn command signal is outputted to the servo amplifier, the turn command signal being composed to make the servo amplifier output a speed command signal to the servo motor, the speed command signal specifying speed 0; the servo motor has a torque control function which holds the ship propulsion unit in a specified position by generating a holding torque opposing an external potential applied to the ship propulsion unit in the case where the external potential does not exceed a predetermined limit value; characterized in that said servo motor has a built-in encoder for detecting its own state, said servo motor, while turning the ship propulsion unit by being driven at a speed specified by a speed command signal received from the servo amplifier, outputting feed-back information on its own speed and electric current to said servo amplifier so that its operating state may be kept in agreement with said command signal from said servo amplifier by collecting information on its own state from said built-in detector and an electric current limit predetermined as a torque limit; and a torque limit protection function which, when the ship propulsion unit is subjected to an external potential exceeding said predetermined limit value, does not allow an electric current exceeding said electric current limit to flow through the servo motor so allowing the ship propulsion unit to turn in the direction of the external potential so that damage to the ship propulsion unit is prevented. a first operating unit for inputting turning position setting information specifying a position to which a ship propulsion unit is to be turned; a first control unit for outputting a turn command signal specifying, based on the turning position setting information inputted from the first operating unit, a direction in which the ship propulsion unit is to be turned and an angle by which the ship propulsion unit is to be turned; a servo amplifier for outputting a speed command signal based on the turn command signal from the first control unit; a servo motor which is engaged with a turning ring provided in the ship propulsion unit rotatably about the turning shaft; and a tracking transmitter for detecting a turning position of the ship propulsion unit and outputting information on the detected turning position as turning position detection information; wherein control is performed in the first control unit such that the turning position detection information from the tracking transmitter and the turning position setting information inputted from the first operating unit is compared and such that, when there is no deviation between the compared information, a turn command signal is outputted to the servo amplifier, the turn command signal being composed to make the servo amplifier output a speed command signal to the servo motor, the speed command signal specifying speed 0; the servo motor has a torque control function which holds the ship propulsion unit in a specified position by generating a holding torque opposing an external potential applied to the ship propulsion unit in the case where the external potential does not exceed a predetermined limit value; characterized in that said servo motor has a built-in encoder for detecting its own state, said servo motor, while turning the ship propulsion unit by being driven at a speed specified by a speed command signal received from the servo amplifier, outputting feed-back information on its own speed and electric current to said servo amplifier so that its operating state may be kept in agreement with said command signal from said servo amplifier by collecting information on its own state from said built-in detector and an electric current limit predetermined as a torque limit; and a torque limit protection function which, when the ship propulsion unit is subjected to an external potential exceeding said predetermined limit value, does not allow an electric current exceeding said electric current limit to flow through the servo motor so allowing the ship propulsion unit to turn in the direction of the external potential so that damage to the ship propulsion unit is prevented.