PURPOSE: To prevent an accident in the compressor of a steam compression type refrigerator by an arrangement wherein a signal indicative of mixture of the air is delivered from the contact of a timer to a controller when a refrigerant circuit is mixed with the air and an alarm is actuated by the controller.

CONSTITUTION: When a refrigerant circuit 12 is mixed with the air and an oxygen level for lowering the sensor output voltage below a reference voltage level is reached, output from a comparator 54 is switched to '1' and a transistor TR 60 is turned on to energize a relay 64. A relay contact 66 is thereby closed but when a timer 68 is time out, contact 70 of the timer is closed and a signal indicative of mixture of the air is delivered to a refrigerator controller 32. In response to a signal obtained from open contacts 66, 70 connected in series, the controller 32 actuates an audibly or/and visually recognizable alarm 72 immediately.