A heat pump system which comprises a main heat pump circuit filled with non-azeotropic mixed coolant and including a compressor, a utility-side heat exchanger, a throttling device, a source-side heat exchanger, etc.,; a fractionating separator having an upper end fluid-connected with an exit side of the utility-side heat exchanger; a reservoir disposed beneath the fractionating separator and having a bottom fluid-connected through a shut-off valve with a low pressure piping on an inlet side of either the source-side heat exchanger or the utility-side heat exchanger; and a coolant ejector disposed between the compressor and the utility-side heat exchanger, wherein a gaseous medium generated from the fractionating separator when the reservoir is heated is guided to a suction port of the coolant ejector to flow into the main heat pump circuit.