A hot water generator comprising: an outdoor unit that includes therein a compressor and a heat exchanger, the heat exchanger being configured to exchange heat between a refrigerant and air; and a hydro unit including a water heat exchanger that exchanges heat between water and the refrigerant flowing in from the outdoor unit, a rectangular housing that has a front face, a rear face, a top face, a bottom face, and a pair of side faces, and an expansion tank that has a cylindrical shape, the expansion tank having a length dimension in a centerline direction larger than diameter dimension, and being accommodated in the housing with the centerline directed toward the pair of side faces, characterized in that the expansion tank includes a cylindrical body, and a convex part that protrudes outward in a radial direction of the cylindrical body and extends in a circumferential direction of the cylindrical body, and in that the hot water generator further comprises a fixing band that has a hole configured to be interdigitated with the convex part, and fixes the expansion tank to a rear plate that forms the rear face of the housing, in a manner that the fixing band extends on the cylindrical body from above to below the cylindrical body. an outdoor unit that includes therein a compressor and a heat exchanger, the heat exchanger being configured to exchange heat between a refrigerant and air; and a hydro unit including a water heat exchanger that exchanges heat between water and the refrigerant flowing in from the outdoor unit, a rectangular housing that has a front face, a rear face, a top face, a bottom face, and a pair of side faces, and an expansion tank that has a cylindrical shape, the expansion tank having a length dimension in a centerline direction larger than diameter dimension, and being accommodated in the housing with the centerline directed toward the pair of side faces, characterized in that the expansion tank includes a cylindrical body, and a convex part that protrudes outward in a radial direction of the cylindrical body and extends in a circumferential direction of the cylindrical body, and in that the hot water generator further comprises a fixing band that has a hole configured to be interdigitated with the convex part, and fixes the expansion tank to a rear plate that forms the rear face of the housing, in a manner that the fixing band extends on the cylindrical body from above to below the cylindrical body. a water heat exchanger that exchanges heat between water and the refrigerant flowing in from the outdoor unit, a rectangular housing that has a front face, a rear face, a top face, a bottom face, and a pair of side faces, and an expansion tank that has a cylindrical shape, the expansion tank having a length dimension in a centerline direction larger than diameter dimension, and being accommodated in the housing with the centerline directed toward the pair of side faces, characterized in that the expansion tank includes a cylindrical body, and a convex part that protrudes outward in a radial direction of the cylindrical body and extends in a circumferential direction of the cylindrical body, and in that the hot water generator further comprises a fixing band that has a hole configured to be interdigitated with the convex part, and fixes the expansion tank to a rear plate that forms the rear face of the housing, in a manner that the fixing band extends on the cylindrical body from above to below the cylindrical body.