To provide a liquid storage unit capable of improving a heat insulation effect in the vicinity of a joint part of a member constituting a liquid storage tank, and to provide a water heater.

A liquid storage tank 3 includes a cylindrical body part 31, and a mirror part connected to the body part 31, and is structured such that a joint part 31a between the body part 31 and the mirror part is located at a portion projected toward the outside of the liquid storage tank 3, normal heat insulation materials 21, 22 are arranged by being divided in the vertical direction of the liquid storage tank by including a boundary part B at a height position different from that of the joint part 31a, and a vacuum heat insulation material 40 is arranged to position the upper end 41 on an upper side relative to the boundary part B.