To defrost an evaporator by heating the evaporator with a discharge gas from a compressor and defrost a zone adjacent to a discharge opening of the evaporator by carrying out an air blowing operation for a given time in a low speed operation mode.

An evaporator 11 is heated by a hot gas discharged from a compressor 6 so that a frost stuck to fins is melted and drips down in a drip pan 16 while a drain water is prevented from being frozen. A usual defrosting control is carried out until a surface temperature Te of the evaporator 11 reaches a given temperature T1 and when the surface temperature Te reaches the temperature T1, an evaporator fan 18 starts a low speed operation thereof and this low speed operation lasts until the surface temperature Te of the evaporator reaches a temperature T2. Due to such a construction, an air having a temperature higher than the given temperature T1 is supplied to an area adJacent to a discharge opening of the evaporator 11 so that an abnormal elevation of the inner temperature can be prevented. Particularly, a frost which sticks to a discharge opening 20 of a duct and a grill can be removed.