Abstract: A heat pump system adapted to use a non-azeotropic refrigerant mixture preferably containing a high boiling temperature refrigerant as a primary component thereof is given an improved heat transfer efficiency and its pressure loss is minimized by arranging a refrigerant extracting means for removing the refrigerant stagnating in the system in an unfavorable state, in a vapor state in a condenser or in liquid state in an evaporator and returning the removed refrigerant into a refrigerant circulating passage of the system; and by constructing an evaporator of a flooded type at least at a portion adjacent to a refrigerant outlet portion of the evaporator.