To provide a device and a method for dehumidifying gas capable of keeping relative humidity of gas after dehumidification in 20-30% level and capable of continuously dehumidifying without changing over flow passage of gas (air or the like).

This device is a heat pump type dehumidifying device including an evaporator of coolant, a compressor, a condenser and a restriction mechanism. The condenser, the evaporator, and a dehumidified gas sending out opening are provided in order from a suction opening along an air flow passage. After temperature of air from the suction opening is raised by heat exchange in the condenser, the air keeps contact on surface of the evaporator to prevent frost formation and is blown out from the sending out opening with humidity in the air condensed and removed. This method is to continuously dehumidify without changing an air flow passage with keeping temperature of coolant in the evaporator 0°C or less with using the device.