PURPOSE: To avoid the effect of bending and twist, by the measurement of tension with piston after converting it into hydraulic pressure.

CONSTITUTION: The piston contained in the cylinder 5 via the cover 11 consists of the slide part 6 and the piston rod 7, and the sectional area of the piston rod 7 is smaller than that of the slide part 6. The room 9 to contain the pressure detection element 13 is formed at the slide part 6, and the room 9, cylinder 5 and the gap 8 with the piston rod 7 are connected through the connection path 10, and liquid is filled in the gap. The cables 1, 2 to detect the tension are connected to the connection parts 3, 4 of the cylinder 5 and the piston rod 7 to detect the pressure of the liquid in the room 9, allowing to detect the tension of the cables 1 and 2.