A jet-propulsion watercraft, comprises a steering position sensor for detecting a steering operation of a steering operation means. A steering means adapted to be subjected to a steering assist mode control, when a signal indicating that the steering position sensor has detected the steering operation is received from the steering position sensor. A control device that gives a signal to the steering position sensor and receives a signal indicating that the steering operation means is or is not steered from the steering position sensor, and the signal from the steering position sensor is set equal in level to the signal given from the control device to the steering position sensor when the steering operation means is not steered.