PURPOSE: To prevent a moisture choke or an occurrence of hydrolysis of oil in advance by a method wherein when a pressure reducing pump means is fixed to an exhausting port and the pump is driven, an inner pressure of an oil filling tank is reduced, moisture content in the oil of a freezer is evaporated and moisture contained in the oil of the freezer is removed.

CONSTITUTION: A pressure reducing pump means 500 is connected to an exhausting port 5 through an exhausting pipe 51. Each of closed valves 30, 50 is released under a condition in which a supplementing valve 41 and a uniform pressure valve 43 are closed and then the pressure reducing pump 500 is driven, oil is filled within an oil filling tank 4 from an external oil tank 300. Then, a closing valve 30 in a conduit 31. is closed and the pressure reducing pump means 500 is further driven toward a high vacuum side, resulting in that an internal pressure within the oil filling tank 4 is further reduced, moisture contained in the oil of the freezer is evaporated and then discharged out of the machine. At this time, a heating source 6 is concurrently and electrically energized to enable evaporation of moisture from the oil to be promoted.