A heat exchanger system comprising: a heat exchanger configured to allow a fluid to flow therethrough and perform heat exchange; and a valve (2; 202) including an external inlet (2a; 302a) and an external outlet (2b; 302b), both of which are connected to an external fluid circuit configured to allow the fluid to flow therethrough, a supply port (2c; 302c) configured to allow the fluid to be supplied to the heat exchanger, and a return port (2d; 302d) configured to receive the fluid that has passed through the heat exchanger; characterized in that the valve (2; 202) is configured to be switchable between: a first state in which the external inlet (2a; 302a) and the supply port (2c; 302c) are connected to each other, and the external outlet (2b; 302b) and the return port (2d; 302d) are connected to each other; and a second state in which at least one of the supply port (2c; 302c) and the return port (2d; 302d) is shut off, and the external inlet (2a; 302a) and the external outlet (2b; 302b) are connected to each other; and the valve (2; 202) includes: only one valve body (25; 210) configured to move to contact a first valve seat (23a; 323a) on a first surface (25a; 211a) in a moving direction of the valve body (25; 210) so as to switch to the first state and to contact a second valve seat (24a; 324a) on a second surface (25b; 211b) in the moving direction of the valve body (25; 210) so as to switch to the second state; a rod urged toward the first valve seat (23a; 323a) by a spring member; and a temperature-sensitive drive (29; 212) provided at a tip of the rod and configured to move the valve body (25; 210) according to a temperature of the fluid; and the only one valve body (25; 210) having the first surface (25a; 211a) and the second surface (25b; 211b) is fixed to the temperature-sensitive drive (29; 212). a heat exchanger configured to allow a fluid to flow therethrough and perform heat exchange; and a valve (2; 202) including an external inlet (2a; 302a) and an external outlet (2b; 302b), both of which are connected to an external fluid circuit configured to allow the fluid to flow therethrough, a supply port (2c; 302c) configured to allow the fluid to be supplied to the heat exchanger, and a return port (2d; 302d) configured to receive the fluid that has passed through the heat exchanger; characterized in that the valve (2; 202) is configured to be switchable between: a first state in which the external inlet (2a; 302a) and the supply port (2c; 302c) are connected to each other, and the external outlet (2b; 302b) and the return port (2d; 302d) are connected to each other; and a second state in which at least one of the supply port (2c; 302c) and the return port (2d; 302d) is shut off, and the external inlet (2a; 302a) and the external outlet (2b; 302b) are connected to each other; and a first state in which the external inlet (2a; 302a) and the supply port (2c; 302c) are connected to each other, and the external outlet (2b; 302b) and the return port (2d; 302d) are connected to each other; and a second state in which at least one of the supply port (2c; 302c) and the return port (2d; 302d) is shut off, and the external inlet (2a; 302a) and the external outlet (2b; 302b) are connected to each other; and the valve (2; 202) includes: only one valve body (25; 210) configured to move to contact a first valve seat (23a; 323a) on a first surface (25a; 211a) in a moving direction of the valve body (25; 210) so as to switch to the first state and to contact a second valve seat (24a; 324a) on a second surface (25b; 211b) in the moving direction of the valve body (25; 210) so as to switch to the second state; a rod urged toward the first valve seat (23a; 323a) by a spring member; and a temperature-sensitive drive (29; 212) provided at a tip of the rod and configured to move the valve body (25; 210) according to a temperature of the fluid; and only one valve body (25; 210) configured to move to contact a first valve seat (23a; 323a) on a first surface (25a; 211a) in a moving direction of the valve body (25; 210) so as to switch to the first state and to contact a second valve seat (24a; 324a) on a second surface (25b; 211b) in the moving direction of the valve body (25; 210) so as to switch to the second state; a rod urged toward the first valve seat (23a; 323a) by a spring member; and a temperature-sensitive drive (29; 212) provided at a tip of the rod and configured to move the valve body (25; 210) according to a temperature of the fluid; and the only one valve body (25; 210) having the first surface (25a; 211a) and the second surface (25b; 211b) is fixed to the temperature-sensitive drive (29; 212).