An air-conditioning apparatus (100, 101), comprising: a refrigerant circuit that is a circuit through which a heat source side refrigerant flows, the refrigerant circuit connecting a compressor, a heat source side heat exchanger, an expansion device, and a plurality of heat exchangers related to heat medium that exchange heat between the heat source side refrigerant and a heat medium different to the heat source side refrigerant; and a heat medium circuit that is a circuit through which the heat medium is made to circulate, the heat medium circuit connecting the plurality of heat exchangers related to heat medium, a plurality of pumps, a plurality of use side heat exchangers, a plurality of first heat medium flow switching devices that allow an outlet side passage of each of the use side heat exchangers to be in communication with the heat exchangers related to heat medium selectively, a plurality of second heat medium flow switching devices that allow an inlet side passage of each of the use side heat exchangers to be in communication with the heat exchangers related to heat medium selectively, and a plurality of heat medium flow control devices that each control a flow rate of the heat medium flowing in the corresponding use side heat exchanger, wherein the air-conditioning apparatus is capable of performing a cooling and heating mixed operation mode, characterized in that the refrigerant circuit has a plurality of expansion devices, first and second on-off devices (51, 52) that open and close the heat medium circuit is provided to a portion of the heat medium circuit that is on an upstream side of each heat medium flow control device and on a downstream side of the corresponding second heat medium flow switching device, a backflow prevention device (40, 43) that is capable of restricting the heat medium from flowing from each first heat medium flow switching device to the corresponding heat medium flow control device is provided to a portion of the heat medium circuit that is on a downstream side of the heat medium flow control device and on an upstream side of the first heat medium flow switching device, wherein the first on-off device is provided to a portion of the heat medium circuit that is on the upstream side of the corresponding use side heat exchanger, and the second on-off device is provided to a portion of the heat medium circuit that is on the downstream side of the corresponding use side heat exchanger. a refrigerant circuit that is a circuit through which a heat source side refrigerant flows, the refrigerant circuit connecting a compressor, a heat source side heat exchanger, an expansion device, and a plurality of heat exchangers related to heat medium that exchange heat between the heat source side refrigerant and a heat medium different to the heat source side refrigerant; and a heat medium circuit that is a circuit through which the heat medium is made to circulate, the heat medium circuit connecting the plurality of heat exchangers related to heat medium, a plurality of pumps, a plurality of use side heat exchangers, a plurality of first heat medium flow switching devices that allow an outlet side passage of each of the use side heat exchangers to be in communication with the heat exchangers related to heat medium selectively, a plurality of second heat medium flow switching devices that allow an inlet side passage of each of the use side heat exchangers to be in communication with the heat exchangers related to heat medium selectively, and a plurality of heat medium flow control devices that each control a flow rate of the heat medium flowing in the corresponding use side heat exchanger, wherein the air-conditioning apparatus is capable of performing a cooling and heating mixed operation mode, characterized in that the refrigerant circuit has a plurality of expansion devices, first and second on-off devices (51, 52) that open and close the heat medium circuit is provided to a portion of the heat medium circuit that is on an upstream side of each heat medium flow control device and on a downstream side of the corresponding second heat medium flow switching device, a backflow prevention device (40, 43) that is capable of restricting the heat medium from flowing from each first heat medium flow switching device to the corresponding heat medium flow control device is provided to a portion of the heat medium circuit that is on a downstream side of the heat medium flow control device and on an upstream side of the first heat medium flow switching device, wherein the first on-off device is provided to a portion of the heat medium circuit that is on the upstream side of the corresponding use side heat exchanger, and the second on-off device is provided to a portion of the heat medium circuit that is on the downstream side of the corresponding use side heat exchanger.