A valve gear comprising: a valve main body which includes a valve chamber communicating with an introduction channel for introducing a refrigerant and a release channel for releasing the refrigerant to an external portion; a valve seat which is formed at an end portion of the introduction channel in the valve chamber side; a valve body which is disposed within the valve chamber so as to be movable forward and backward with respect to the valve seat; and a valve closing spring which biases the valve body toward the valve seat, wherein in a case where the refrigerant flowing into the valve gear through the introduction channel is equal to or less than a fixed pressure, the valve body is seated on the valve seat on the basis of a biasing force of the valve closing spring, thereby shutting off a flow channel from the introduction channel to the release channel, and in a case where the refrigerant flowing into the valve gear from the introduction channel becomes greater than the fixed pressure, the valve body moves backward from the valve seat against the biasing force of the valve closing spring, thereby opening the flow channel from the introduction channel to the release channel and releasing the refrigerant to the external portion, wherein the valve seat is constructed by a wall body which rises up vertically upward from an opening edge portion of the upper end of the introduction channel and has a fixed height, and the wall body extends so as to surround the opening of the upper surface of the introduction channel, wherein an upper end portion of the wall body has a semicircular dome-like cross sectional shape, and the valve body is disposed to come into contact with a top of the wall body and to close the introduction channel when closing the valve, wherein the valve body is made of a soft material containing a resin or a rubber as a major component, and wherein depressed portions (22, 23)are provided in a surface of the valve body in a side facing the introduction channel, the depressed portions (22, 23) including an outer groove which is formed in an outer side of the valve seat in such a manner as to surround the valve seat and an inner hole which is formed in an inner side of the valve seat. a valve main body which includes a valve chamber communicating with an introduction channel for introducing a refrigerant and a release channel for releasing the refrigerant to an external portion; a valve seat which is formed at an end portion of the introduction channel in the valve chamber side; a valve body which is disposed within the valve chamber so as to be movable forward and backward with respect to the valve seat; and a valve closing spring which biases the valve body toward the valve seat, wherein in a case where the refrigerant flowing into the valve gear through the introduction channel is equal to or less than a fixed pressure, the valve body is seated on the valve seat on the basis of a biasing force of the valve closing spring, thereby shutting off a flow channel from the introduction channel to the release channel, and in a case where the refrigerant flowing into the valve gear from the introduction channel becomes greater than the fixed pressure, the valve body moves backward from the valve seat against the biasing force of the valve closing spring, thereby opening the flow channel from the introduction channel to the release channel and releasing the refrigerant to the external portion, wherein the valve seat is constructed by a wall body which rises up vertically upward from an opening edge portion of the upper end of the introduction channel and has a fixed height, and the wall body extends so as to surround the opening of the upper surface of the introduction channel, wherein an upper end portion of the wall body has a semicircular dome-like cross sectional shape, and the valve body is disposed to come into contact with a top of the wall body and to close the introduction channel when closing the valve, wherein the valve body is made of a soft material containing a resin or a rubber as a major component, and wherein depressed portions (22, 23)are provided in a surface of the valve body in a side facing the introduction channel, the depressed portions (22, 23) including an outer groove which is formed in an outer side of the valve seat in such a manner as to surround the valve seat and an inner hole which is formed in an inner side of the valve seat.