Relief Valve Device

A relief valve device comprising:

• a housing provided with a valve passage through which a primary port and a secondary port are connected to each other;
• a valve body which is disposed at a closed position where the valve passage is closed and is displaceable in an opening direction toward an open position where the valve passage is opened, the valve body being provided so as to receive resultant of pressing forces of a primary pressure of fluid guided through the primary port in the opening direction in a state where the valve body is disposed at a position closer to the open position including a fluid driving start position between the closed position and the open position, and so as to receive smaller resultant of the pressing forces of the primary pressure in the opening direction in a state where the valve body is disposed at a position closer to the closed position than to the fluid driving start position than the resultant of the pressing forces received by the valve body in the state where the valve body is disposed at the position closer to the open position including the fluid driving start position;
• a pressing means for pressing the valve body in the opening direction;
• and
• a displacement preventing means which is made of a material melted when its temperature reaches a predetermined melting point or higher, the displacement preventing means being provided in a space opened to an outside space and supporting the valve body disposed at the closed position to prevent displacement of the valve body in the opening direction.