An air-conditioning apparatus (100, 100A), comprising: a refrigeration cycle including a refrigerant circuit for circulating a refrigerant, the refrigerant circuit being constituted by connecting with pipes (4, 4a, 4b) a compressor that sends out a combustible refrigerant, a refrigerant flow switching device (11, 18a, 18b) configured to switch circulation paths of the refrigerant, a heat source side heat exchanger configured to exchange heat of the refrigerant, a refrigerant expansion device (16a, 16b, 16c) configured to control a pressure of the refrigerant, and a heat exchanger related to heat medium (15a, 15b) capable of exchanging heat between the refrigerant and a heat medium that is different from the refrigerant; and a heat medium side circuit constituted by a heat medium circulating circuit that is constructed by connecting a heat medium sending device (21a, 21b) configured to circulate the heat medium pertaining to heat exchange of the heat exchanger related to heat medium (15a, 15b), and a use side heat exchanger (26a, 26b, 26c, 26d) exchanging heat between the heat medium and air related to a space to be air-conditioned, with pipes (5, 5a, 5b, 5c, 5d), wherein at least the compressor, the refrigerant flow switching device, the heat source side heat exchanger are housed in an outdoor unit, at least the heat exchanger related to heat medium (15a, 15b) and the refrigerant expansion device (16a, 16b, 16c) are housed in a heat medium relay unit (3, 3a, 3b), and the use side heat exchanger (26a, 26b, 26c, 26d) is housed in an indoor unit (2, 2a, 2b, 2c, 2d), each of the outdoor unit, the heat medium relay unit (3, 3a, 3b), and the indoor unit (2, 2a, 2b, 2c, 2d) being separately formed and being allowed to be disposed at separate positions, and a housing of the heat medium relay unit (3, 3a, 3b) includes an opening (61; 61A.61B) allowing ventilation between a housing space of the heat exchanger related to heat medium and a space other than the housing, and the air-conditioning apparatus being characterized in that: the air-conditioning apparatus further comprises: a relay-unit air-sending device that sends air; and a controller configured to perform a control operation of the relay-unit air-sending device such that refrigerant concentration inside the housing is maintained under a predetermined concentration, wherein the controller operates the relay-unit air-sending device in order to maintain the refrigerant concentration under the predetermined concentration even when the compressor of the outdoor unit is in a suspended state. a refrigeration cycle including a refrigerant circuit for circulating a refrigerant, the refrigerant circuit being constituted by connecting with pipes (4, 4a, 4b) a compressor that sends out a combustible refrigerant, a refrigerant flow switching device (11, 18a, 18b) configured to switch circulation paths of the refrigerant, a heat source side heat exchanger configured to exchange heat of the refrigerant, a refrigerant expansion device (16a, 16b, 16c) configured to control a pressure of the refrigerant, and a heat exchanger related to heat medium (15a, 15b) capable of exchanging heat between the refrigerant and a heat medium that is different from the refrigerant; and a heat medium side circuit constituted by a heat medium circulating circuit that is constructed by connecting a heat medium sending device (21a, 21b) configured to circulate the heat medium pertaining to heat exchange of the heat exchanger related to heat medium (15a, 15b), and a use side heat exchanger (26a, 26b, 26c, 26d) exchanging heat between the heat medium and air related to a space to be air-conditioned, with pipes (5, 5a, 5b, 5c, 5d), wherein at least the compressor, the refrigerant flow switching device, the heat source side heat exchanger are housed in an outdoor unit, at least the heat exchanger related to heat medium (15a, 15b) and the refrigerant expansion device (16a, 16b, 16c) are housed in a heat medium relay unit (3, 3a, 3b), and the use side heat exchanger (26a, 26b, 26c, 26d) is housed in an indoor unit (2, 2a, 2b, 2c, 2d), each of the outdoor unit, the heat medium relay unit (3, 3a, 3b), and the indoor unit (2, 2a, 2b, 2c, 2d) being separately formed and being allowed to be disposed at separate positions, and a housing of the heat medium relay unit (3, 3a, 3b) includes an opening (61; 61A.61B) allowing ventilation between a housing space of the heat exchanger related to heat medium and a space other than the housing, and the air-conditioning apparatus being characterized in that: the air-conditioning apparatus further comprises: a relay-unit air-sending device that sends air; and a controller configured to perform a control operation of the relay-unit air-sending device such that refrigerant concentration inside the housing is maintained under a predetermined concentration, wherein the controller operates the relay-unit air-sending device in order to maintain the refrigerant concentration under the predetermined concentration even when the compressor of the outdoor unit is in a suspended state. a relay-unit air-sending device that sends air; and a controller configured to perform a control operation of the relay-unit air-sending device such that refrigerant concentration inside the housing is maintained under a predetermined concentration, wherein the controller operates the relay-unit air-sending device in order to maintain the refrigerant concentration under the predetermined concentration even when the compressor of the outdoor unit is in a suspended state.