To provide a steering auxiliary implement with which the steerability of one-rudder type ship to be improved without losing the balance of the ship.

The steering auxiliary implement 1 includes auxiliary rudder fitting plates 3 which are fitted to a plate-like main rudder 2 of the ship so as to extend in the right-to-left direction from both sides of the main rudder 2 when viewed from the aft side of a ship, and a pair of plate-like auxiliary rudders 4, 4 provided on right and left ends of the auxiliary rudder fitting plates 3. The auxiliary rudder 4 is parallel to the main rudder 2, and the steerability can be enhanced by ensuring the rudder area by providing the pair of auxiliary rudders 4.