PURPOSE: To separately detect the axial tensions and bending moments generated in a hydrofoil and a ship body by measuring the axial strain and bending strain of the connection member of the hydrofoil by the detection sensor arranged to the connection member.

CONSTITUTION: Load consisting of axial load and bending moment 7 is generated in four connection members 3 connecting a ship body 1 and hydrofoil 2. Strain sensors 4x, 4y and 5, 5 in a lateral direction (x-direction) and an axial direction (y-direction) are bonded to the outsides and insides of the respective connection members 31 at symmetric positions. A bridge circuit inputting the outputs from the respective strain sensors 4x, 4y, 5, 5 adds the outside and inside axial strain outputs of the respective connection members 3 not only to detect the axial tensions generated in the connection members 3 but also to subtract the inside axial strain outputs of the connection members 3 from the outside axial strain outputs thereof to detect the bending moments generated in the connection members 3.