PURPOSE: To generate an axial tension by a method wherein a load cell is arranged in a manner to annually coaxially surround the tip of a bolt through a bridge brought into contact with the nut seat surface of the upper cover of a steam valve, an elongation amount and the temperature of the bolt are measured, and the axial tension is controlled through correction of an elongation amount owing to temperature.

CONSTITUTION: In a state that an upper cover 2 is fixed by means of a nut 4 by utilizing a stud bolt 3 arranged to a steam valve body 1, an annular bridge 5 with which approximately a whole is covered is brought into contact with the upper surface of the upper cover 2 and a load cell 6 is fixed through a bridge 5 in a manner to surround the bolt 3 on the same axis as that of the load cell. A puller 7 joined in a screwed-in state with the bolt 3 is inserted in the inner peripheral part of the load cell 6 and by pressing a piston 9 in a cylinder 8 formed in the upper part of the load cell 6 by means of an oil pressure, an axial tension is exerted on the bolt 3 through the puller 7. The axial tension is corrected by means of a bolt elongation amount measured by an elongation amount measuring apparatus 17 and temperature detected by a temperature sensor 25. Control is carried out through operation of a pump unit 10 according to a deviation between an elongation amount after correction and a set elongation amount.