A heat pump system for a vehicle which uses an evaporator of an air-conditioning case for common use in an air-conditioning mode and in a heat pump mode for cooling and heating so as to reduce weight and manufacturing costs. The heat pump system can be applied without any change in structure of the air-conditioning case, and can be operated even at low temperature without any influence of outdoor temperature because the heat pump system uses waste heat of an engine so as to enhance fuel efficiency and increase heating performance and efficiency.