PURPOSE: To realize more stable sailing by improving a flap controller for controlling strip body rocking especially, as for a hydrofoil craft provided with completely submerged hydrofoils.

CONSTITUTION: A hydrofoil craft provided with completely submerged hydrofoils is provided with a flap controller 24 which can control rocking of a ship body by controlling rudder flaps 2a, 2b and hydrofoil flaps 3a, 3b, 7a, 8a according to detection signals from sensors 10-14, 16, 18, 19, 21 which detect rocking information of the ship body. The flap controller 24 comprises a forward compensation control system 22 which calculates forward compensation gain based on tire detection signals from the sonsors so as to perform flap control and a feedback control system 23 which calculates feedback gain based on the detection signals so as to perform flap control. Thereby stability and riding quality of sailing can be remarkably improved.