A heat treatment device includes: a heating device that heats a treatment object; a cooling device including a cooling room that accommodates the treatment object heated by the heating device and into which a cooling medium used for cooling the treatment object is supplied; a pressurized gas supplier that supplies pressurized gas into the cooling room; a pressure relief valve that communicates internal and external areas of the cooling room with each other when the pressure relief valve is opened; a pressure sensor that measures the pressure inside the cooling room; and a controller that controls the pressure relief valve such that the pressure relief valve is opened when a measurement result of the pressure sensor is higher than or equal to a threshold value.