To provide a power steering device for a boat with an outboard motor which helps to realize easy drive with small steering force without taking waterproofness into consideration. A gear device (4) is provided for driving a link mechanism for turning and steering an outboard motor main body at the rear of the boat body through a cable by a steering wheel (2). A steering torque input to the gear device (4) by the steering wheel (2) is detected by a torque sensor (35) to assist-drive the gear device 4 in the steering direction by an electric motor (27), a helical pinion (25), and a helical wheel (26).