There is provided a relief valve in which a valve body is automatically properly opened/closed by pressure of gas while having a simple device structure, and a device including the relief valve. In a valve-closed state, a first pressure receiving surface portion 4f receives pressure of gas from a valve hole 2j to smoothly open a valve body 4, and in an open position after the valve is opened, a pressure receiving area is enlarged to a second pressure receiving surface portion 4g, and the first pressure receiving surface portion 4f and the second pressure receiving surface portion 4g are surrounded by a peripheral wall portion 4c formed on a back surface of the valve body 4. Thus, the valve body 4 continuously receives the pressure with the pressure of the gas being reduced, thereby allowing the valve-opened state of the valve body 4 to be stably maintained to set pressure.
