PURPOSE: To prevent a valve from being dried, and to remove the troublesome residue of the valve by executing the adjustment of an open pressure by the movement adjustment of an adjusting device by a calculated required stroke value.

CONSTITUTION: A prescribed normalized first pressure P1 is loaded to a path 16, a valve 10 is opened by adjusting an adjusting member 26, the adjusting member 26 is screwed into a cage 12, the valve 10 is just closed, and the position of the adjusting member 26 in the casing is established. Then, a prescribed normalized second inspection pressure P2 is loaded to the path 16, the valve 10 is opened, the adjusting member 26 is deeply screwed into the casing 12 until the path 16 is just closed from a position detected by the first inspection pressure P1, and the relative position of the adjusting member 26 to the casing 12 is newly detected. Then, the adjustment interval or stroke of the adjusting member 26 is detected from two positions detected for the first and second test pressures, and a required stroke value for the adjustment of a desired prescribed open pressure P3 is calculated.