A valve unit (100, 100a) used for a heat-pump system, comprising: at least one liquid refrigerant pipe portion; at least one gas refrigerant pipe portion; at least one liquid control valve disposed in the liquid refrigerant pipe portion; at least one gas control valve disposed in the gas refrigerant pipe portion; a casing accommodating at least the liquid control valve and the gas control valve; an air-discharge mechanism configured to operate to discharge an air in an internal space of the casing to an external space outside the casing; a sensor configured to detect a concentration of the refrigerant in the air in the casing; and a controller configured to determine that a refrigerant leakage in the casing has occurred when the detected concentration is equal to or greater than a detection value threshold, and control the air-discharge mechanism to start operation when the refrigerant leakage has occurred, wherein: the casing is formed with a plurality of pipe apertures; an intake opening and a discharge opening are formed in the casing; and the air-discharge mechanism includes a fan configured to draw the air from the internal space towards the external space, characterized in that: a suction port or a discharge port of the fan is connected to the discharge opening of the casing by an air duct, or the fan is mounted to the casing at the discharge opening such that a suction port of the fan faces the internal space of the casing and a discharge port of the fan faces the outside of the casing; the air-discharge mechanism further includes a check air damper configured to allow an air to flow from an outside of the casing towards the internal space through the intake opening when the fan is in operation; and the valve unit further comprises insulators applied to the casing such that an internal space of the casing is substantially isolated from an outer space surrounding the casing at least when the air-discharge mechanism is not in operation. at least one liquid refrigerant pipe portion; at least one gas refrigerant pipe portion; at least one liquid control valve disposed in the liquid refrigerant pipe portion; at least one gas control valve disposed in the gas refrigerant pipe portion; a casing accommodating at least the liquid control valve and the gas control valve; an air-discharge mechanism configured to operate to discharge an air in an internal space of the casing to an external space outside the casing; a sensor configured to detect a concentration of the refrigerant in the air in the casing; and a controller configured to determine that a refrigerant leakage in the casing has occurred when the detected concentration is equal to or greater than a detection value threshold, and control the air-discharge mechanism to start operation when the refrigerant leakage has occurred, wherein: the casing is formed with a plurality of pipe apertures; an intake opening and a discharge opening are formed in the casing; and the air-discharge mechanism includes a fan configured to draw the air from the internal space towards the external space, characterized in that: a suction port or a discharge port of the fan is connected to the discharge opening of the casing by an air duct, or the fan is mounted to the casing at the discharge opening such that a suction port of the fan faces the internal space of the casing and a discharge port of the fan faces the outside of the casing; the air-discharge mechanism further includes a check air damper configured to allow an air to flow from an outside of the casing towards the internal space through the intake opening when the fan is in operation; and the valve unit further comprises insulators applied to the casing such that an internal space of the casing is substantially isolated from an outer space surrounding the casing at least when the air-discharge mechanism is not in operation. the casing is formed with a plurality of pipe apertures; an intake opening and a discharge opening are formed in the casing; and the air-discharge mechanism includes a fan configured to draw the air from the internal space towards the external space, characterized in that: a suction port or a discharge port of the fan is connected to the discharge opening of the casing by an air duct, or the fan is mounted to the casing at the discharge opening such that a suction port of the fan faces the internal space of the casing and a discharge port of the fan faces the outside of the casing; the air-discharge mechanism further includes a check air damper configured to allow an air to flow from an outside of the casing towards the internal space through the intake opening when the fan is in operation; and the valve unit further comprises insulators applied to the casing such that an internal space of the casing is substantially isolated from an outer space surrounding the casing at least when the air-discharge mechanism is not in operation. a suction port or a discharge port of the fan is connected to the discharge opening of the casing by an air duct, or the fan is mounted to the casing at the discharge opening such that a suction port of the fan faces the internal space of the casing and a discharge port of the fan faces the outside of the casing; the air-discharge mechanism further includes a check air damper configured to allow an air to flow from an outside of the casing towards the internal space through the intake opening when the fan is in operation; and the valve unit further comprises insulators applied to the casing such that an internal space of the casing is substantially isolated from an outer space surrounding the casing at least when the air-discharge mechanism is not in operation.