A method for controlling posture control tabs (21A, 21B) of a marine vessel, the marine vessel including a steering control configured to give instructions for controlling a direction in which a hull of the marine vessel moves, a propulsion device configured to generate a propulsive force to move the hull, a port-side posture control tab and a starboard-side posture control tab mounted on a port side and a starboard side of a stern of the hull, configured to be movable up or down to control a posture of the hull, and a port-side actuator and a starboard-side actuator configured to respectively actuate the port-side posture control tab and the starboard-side posture control tab, the method comprising: acquiring a steering instruction given through the steering control; acquiring a trim position of the propulsion device; and upon acquiring the steering instruction, executing first control to determine an actual trim angle on a basis of the acquired trim position and a pitch angle of the hull, determine an operation direction of the steering control in which an operation force necessary to operate the steering control becomes large by comparing the actual trim angle with a reference angle, determine one of the port-side posture control tab and the starboard side posture control tab, located on a same side as the determined operation direction, as a posture control tab to be actuated downward in a case where a direction of a change in movement of the hull instructed by the steering instruction agrees with the determined operation direction, and control one of the port-side actuator and the starboard-side actuator corresponding to the determined posture control tab so as to move the determined posture control tab downward, wherein the reference angle is determined in advance in consideration of the acquired trim position, the pitch angle of the hull, a vessel speed of the marine vessel, a propeller pitch of a propeller of the propulsion device, a propeller rotation speed of the propeller and a paddle wheel effect due to the propeller. acquiring a steering instruction given through the steering control; acquiring a trim position of the propulsion device; and upon acquiring the steering instruction, executing first control to determine an actual trim angle on a basis of the acquired trim position and a pitch angle of the hull, determine an operation direction of the steering control in which an operation force necessary to operate the steering control becomes large by comparing the actual trim angle with a reference angle, determine one of the port-side posture control tab and the starboard side posture control tab, located on a same side as the determined operation direction, as a posture control tab to be actuated downward in a case where a direction of a change in movement of the hull instructed by the steering instruction agrees with the determined operation direction, and control one of the port-side actuator and the starboard-side actuator corresponding to the determined posture control tab so as to move the determined posture control tab downward, wherein the reference angle is determined in advance in consideration of the acquired trim position, the pitch angle of the hull, a vessel speed of the marine vessel, a propeller pitch of a propeller of the propulsion device, a propeller rotation speed of the propeller and a paddle wheel effect due to the propeller.