To provide a hot water storage tank unit: enabling reduction in an arrangement space of external pipes regardless of an installation position of a heating device for heating tank water; and enabling a maintenance and inspection work of functional components in an outer casing without problem.

Connection ports 75, 76 to which the external pipes for the heating device, connecting the heating device and a hot water storage tank 2, are connected, are formed on one face 3X1 in the X-axis direction of the outer casing 3, when the opposing direction of a set of opposed faces excluding upper and lower faces of the outer casing 3 is the X-axis direction, the opposing direction of the remaining one set of opposed faces is a Y-axis direction, and a height direction is a Z-axis direction. The plurality of functional components 21b, 23b, 24a, 24b, 25, 6 disposed in the outer casing 3 and requiring maintenance and inspection, are disposed along a neutral face 3M parallel to the X-axis direction and the Z-axis direction, positioned on a central section in the Y-axis direction of the outer casing 3 in a state that they are not overlapped to the hot water storage tank 2 and the other functional components in the Y-axis direction.