A heat medium relay unit comprising a primary heat medium side assembly, a secondary heat medium flow path switching device assembly, and a drain pan, the primary heat medium side assembly including a heat exchanger (1a, 1b, 1c, 1d) configured to exchange heat between a primary heat medium and a secondary heat medium, the primary heat medium circulating between the heat medium relay unit and an outdoor unit connected by a pipe, the secondary heat medium circulating between the heat medium relay unit and an indoor unit connected by a pipe; a secondary heat medium sending device (2a, 2b) configured to pump the secondary heat medium for circulating between the heat medium relay unit and the indoor unit; and a primary-side casing portion including side panels (4a, 4b) covering side surfaces of the heat medium relay unit, and lower frames (5c, 5d) that connect between the side panels (4a, 4b) and to which the secondary heat medium sending device (2a, 2b) is attached, the secondary heat medium flow path switching device assembly including a secondary heat medium flow path switching device configured to select or mix the secondary heat medium flowing through a plurality of flow paths, and cause the secondary heat medium to flow into and out of the indoor unit; and a secondary-side casing portion including upper frames (5a, 5b) connecting between the side panels (4a, 4b), characterized in that the drain pan is configured to have width and depth dimensions larger than dimensions of an outer shell of a heat medium relay unit main body, the outer shell including the side panels (4a, 4b), the upper frames (5a, 5b), and the lower frames (5c, 5d), and an upper end surface whose position is higher than that of an upper end surface of the lower frames (5c, 5d). a heat exchanger (1a, 1b, 1c, 1d) configured to exchange heat between a primary heat medium and a secondary heat medium, the primary heat medium circulating between the heat medium relay unit and an outdoor unit connected by a pipe, the secondary heat medium circulating between the heat medium relay unit and an indoor unit connected by a pipe; a secondary heat medium sending device (2a, 2b) configured to pump the secondary heat medium for circulating between the heat medium relay unit and the indoor unit; and a primary-side casing portion including side panels (4a, 4b) covering side surfaces of the heat medium relay unit, and lower frames (5c, 5d) that connect between the side panels (4a, 4b) and to which the secondary heat medium sending device (2a, 2b) is attached, a secondary heat medium flow path switching device configured to select or mix the secondary heat medium flowing through a plurality of flow paths, and cause the secondary heat medium to flow into and out of the indoor unit; and a secondary-side casing portion including upper frames (5a, 5b) connecting between the side panels (4a, 4b), characterized in that the drain pan is configured to have width and depth dimensions larger than dimensions of an outer shell of a heat medium relay unit main body, the outer shell including the side panels (4a, 4b), the upper frames (5a, 5b), and the lower frames (5c, 5d), and an upper end surface whose position is higher than that of an upper end surface of the lower frames (5c, 5d).