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 . <EOS>