A water tank is provided to prevent deformation of the water tank due to water pressure inside the water tank or soil pressure exerted from the outside the water tank by using a steel plate having a reinforcement on the opposite sides of the water tank. A water tank having opposite sidewalls(11), wherein the water tank is made of corrugated steel pipe in a cylindrical shape and the opposite sidewalls are made of a steel plate(111), the water tank comprises a reinforcement(2), and a reinforcement bar(5). The reinforcement is capable of enduring water pressure inside the water tank or soil pressure exerted from the outside. The reinforcement bar is installed between opposite ends of an inner wall of the water tank and opposite sidewall, so as to support the opposite sidewall.
