PURPOSE: To keep off any early blowoff due to a leakage, by installing a pilot valve in the point midway in a connecting pipe made up of making boiler pressure introducible in a bellows.

CONSTITUTION: A pilot valve body 21 around a pilot valve 2 is set so as to be opened or closed somewhere about 9kg/cm2f in advance. If a stop valve 35 is opened in advance, the pilot valve body 21 still remains lowered in the process that pressure in a boiler 18 is going up, whereby boiler pressure is led into a chamber 13 by a connecting pipe 36, and a valve body 10 is completely closed by downward motion in a main spring 8 and the chamber 13. When the boiler pressure goes up and reaches somewhere about 9kg/cm2f, the pilot valve body 21 is pulled up and pressure in the chamber 13 escapes from a blow port 23 by way of a connecting pipe 16 so that the valve body 10 is opened from the locked state, and when the boiler pressure reaches the point of about 10kg/cm2f, the valve is opened.