A rudder control device for a boat can include a handle position sensor for detecting an operation angle of a handle. An engine control unit can be provided for receiving a detection signal from the handle position sensor. A motor can be provided for receiving a control signal from the engine control unit to drive a steering member to a predetermined rotation angle corresponding to the operation angle. The engine control unit can receive a signal from a speed sensor for detecting a boat speed to control an upper limit of the rotation angle to be smaller when the boat speed is higher than a predetermined value than when not.