Provided are a relief valve in which a valve body automatically opens and closes automatically by gas pressure and a device including the relief valve while having a simple device structure. When the valve body 4 is in the closed state, the first pressure receiving surface portion 4f receives the gas pressure from the valve hole 2j and smoothly opens, and in the opened position after the valve opening, the pressure receiving area becomes the second pressure receiving surface portion 4g. Since the first pressure receiving surface portion 4f and the second pressure receiving surface portion 4g are surrounded by the peripheral wall portion 4c formed on the back surface of the valve body 4, the pressure of the gas continues to be reduced while decreasing. Thereby, the valve opening state of the valve body 4 can be stably maintained up to the set pressure. [Selection] Figure 2簡易な装置構造でありながら、弁体が気体の圧力によって自動で適切に開閉動作するリリーフ弁とそれを備える機器を提供する。弁体４が、閉弁状態では第１の受圧面部４ｆにより弁孔２ｊから気体の圧力を受けてスムーズに開弁し、開弁後の開位置では、受圧面積が第２の受圧面部４ｇにまで拡大するとともに、第１の受圧面部４ｆと第２の受圧面部４ｇが弁体４の裏面に形成した周壁部４ｃに取り囲まれていることから、気体の圧力は減少しつつも圧力を受け続けることで、設定圧力まで弁体４の開弁状態を安定して維持することができる。【選択図】図２
