A high-lift twin rudder system, wherein a pair of high-lift rudders (1) and (2) having top end plates (6) and (7) at the top end part of rubber blades (4) and (5) and bottom end plates (8) and (9) at the bottom end part thereof are disposed at the rear of a pusher propeller (3), fins (10) and (11) having specific chord lengths from generally leading edge parts rearward are provided on the inboard sides of the rudder blades (4) and (5) at approximately the same level as the axis of the pusher propeller (3), the fin (10) of one rudder blade (4) opposed to the board side where the pusher propeller blades are rotated upward has an attitude forming such an angle of attach at which the ratio of a thrust in forward direction caused by the wake flow behind the pusher propeller having an upward component of the flow to a drag becomes maximum and the fin (11) of the other rudder blade (5) opposed to the board side where the pusher propeller blades are rotated downward has an attitude forming such an angle of attach at which the ratio of a thrust in forward direction caused by the wake flow behind the pusher propeller having a downward component of the flow to a drag becomes maximum, and the chord lengths of the rudder blades (4) and (5) are formed to be 60 to 45% of the diameter of the pusher propeller.