To provide a water mattress device which can be folded smoothly. CONSTITUTION: The device has a base member 41 which is a rectangular board made of a hard material, a folding part on the base member 41 to fold the base member 41 at least in the longitudinal direction, and a frame 44 mounted around the base member 41 which forms a storage room 45 on the top of the base member 41 and which can be folded together with the base member 41. The device also has a water bag 54 inside the storage room which is a bag made of an elastic liquidproof sheet filled with liquid, and a covering material 62 which covers the bottom of the base member 41, the top of the storage room 45, and the outside of the frame 44.