To execute the defrosting while performing heat operation continuously without switching heat operation over to cooling operation by supplying an evaporator with a refrigerant coming out of a condenser and a refrigerant drawn out from a refrigerant drawing means each in separate form.

This frosting preventing device is provided with a directional control valve 3 on one side of the evaporator 1 in heating operation cycle, and a switching plate 2 which rotates at an angle of 90° by the defrosting of the top and bottom of the evaporator is built in the directional control valve 3. Then, a part of the refrigerant in high temperature and high pressure condition is drawn out of the outlet of a compressor 9 by the switching plate 2 through an on-off valve 11 being a refrigerant drawing means and it is made to flow into the first inlet pipe 4 through a bypass pipe 10. Moreover, the refrigerant is made to flow into the second inlet pipe 5 from a condenser 15. Then, the refrigerant from the first inlet pipe 4 is discharged to the first outlet pipe 6, and the refrigerant from the second inlet pipe 5 is discharged to the second outlet pipe 7, and they are supplied each in separate form to the evaporator 1.