A heat pump system comprising a refrigeration cycle device, a usage-side heat medium circuit which circulates usage-side heat medium by a conveying device, and a control device, in which the refrigeration cycle device includes: a main refrigerant circuit formed by sequentially connecting, to one another through a pipe, a compression mechanism composed of a compression rotation element, a usage-side heat exchanger for heating the usage-side heat medium by refrigerant discharged from the compression mechanism, an intermediate heat exchanger, a first expansion device and a heat source-side heat exchanger; and a bypass refrigerant circuit which is branched off from the pipe between the usage-side heat exchanger and the first expansion device and joins up with refrigerant which is in middle of a compression operation of the compression rotation element, the bypass refrigerant circuit provides a second expansion device; and the refrigerant flowing in the bypass refrigerant circuit downstream side from the second expansion device is exchanging heat with the refrigerant flowing in the main refrigerant circuit in the intermediate heat exchanger; wherein the heat pump system includes a heating operation mode in which the usage-side heat medium is heated in the usage-side heat exchanger by refrigerant discharged from the compression mechanism, and the refrigerant flows through the compression mechanism, the usage-side heat exchanger, the intermediate heat exchanger, the first expansion device and the heat source-side heat exchanger in this order in a defrosting operation mode in which the heat source-side heat exchanger is defrosted by refrigerant discharged from the compression mechanism, characterized in that the control device sets the opening degree of the second expansion device to a fully closed opening degree when the heating operation mode is shifted to the defrosting operation mode during execution of the heating operation mode, and the control device stops operation of the conveying device, and stops the heating operation mode after closing fully the opening degree of the second expansion device, and shifts to the defrosting operation mode by stopping of the heating operation mode and setting an opening degree of the first expansion valve to a fully opened opening degree. a refrigeration cycle device, a usage-side heat medium circuit which circulates usage-side heat medium by a conveying device, and a control device, in which the refrigeration cycle device includes: a main refrigerant circuit formed by sequentially connecting, to one another through a pipe, a compression mechanism composed of a compression rotation element, a usage-side heat exchanger for heating the usage-side heat medium by refrigerant discharged from the compression mechanism, an intermediate heat exchanger, a first expansion device and a heat source-side heat exchanger; and a bypass refrigerant circuit which is branched off from the pipe between the usage-side heat exchanger and the first expansion device and joins up with refrigerant which is in middle of a compression operation of the compression rotation element, the bypass refrigerant circuit provides a second expansion device; and the refrigerant flowing in the bypass refrigerant circuit downstream side from the second expansion device is exchanging heat with the refrigerant flowing in the main refrigerant circuit in the intermediate heat exchanger; wherein the heat pump system includes a heating operation mode in which the usage-side heat medium is heated in the usage-side heat exchanger by refrigerant discharged from the compression mechanism, and the refrigerant flows through the compression mechanism, the usage-side heat exchanger, the intermediate heat exchanger, the first expansion device and the heat source-side heat exchanger in this order in a defrosting operation mode in which the heat source-side heat exchanger is defrosted by refrigerant discharged from the compression mechanism, characterized in that the control device sets the opening degree of the second expansion device to a fully closed opening degree when the heating operation mode is shifted to the defrosting operation mode during execution of the heating operation mode, and the control device stops operation of the conveying device, and stops the heating operation mode after closing fully the opening degree of the second expansion device, and shifts to the defrosting operation mode by stopping of the heating operation mode and setting an opening degree of the first expansion valve to a fully opened opening degree.