Heat exchanger and heat pump system, which uses the same

A heat exchanger having a main core portion (6) for effecting a heat exchange between air and a refrigerant, a receiver tank (8), in which a coolant flows, which has passed through the main core portion and a subcool core portion (10) for cooling and liquefying a refrigerant with a gas / liquid mixture that has passed through the holding tank, through a heat exchange with the air, wherein the subcool core portion of many tubes (22), which are arranged along a transverse direction between a left and a right header tank (20) are each arranged along the transverse direction, and in a state that they are parallel to each other and both with the left and to the right header tank in communication; and fins (24) which are arranged between the adjacent tubes.