To provide a water mattress which can support a human body under a uniform pressure without lowering body-supporting performance in an ordinary state that the water mattress is flat by taking a structure that movement of water within a water bag is lessened as much as possibly in a state that a back portion is raised when the water mattress is applied to a Gatch bed (bed and bed boards with motorized/manual adjustment of body position), and is compactly folded and easy to be stored and transported when it is out of use.

The water mattress is obtained by arranging a cushioning member 2 composed of an elastic foam at an internal peripheral edge of an openable and closeable rectangular bag 1 to form a water bag container portion 3, and laying a plurality of water bags 4 parallel in a prescribed direction within the water bag container portion 3, wherein the elastic foam 5 having a volume substantially equal to the internal volume of the water bag 4 is contained in the water bag, and water is enclosed in the water bag 4.