PURPOSE: To change hardness of stuffing by providing stuffing which contains a foam elastic body in bag bodies having gastighteness on one face of a spring unit and shrinking the bag bodies through a process of discharging air in the bag bodies, thereby compressing the foam elastic body.

CONSTITUTION: In a mattress device, by operating an air pump by a switch or the like, air within respective rooms 6a, 6b, 6c formed in an upper stuffing 6 is gradually discharged through first and second pipes 22, 24. Along with discharge of air, the bag bodies 19 constituting the upper stuffing 6 gradually shrink and simultaneously foam elastic body 21 contained in the bag bodies 19 is gradually compressed by being pressed by the bag bodies 19, so that the foam elastic body 21 gradually hardens. In this manner, hardness of the upper stuffing 6 is changed. Accordingly, a user taking this advantage lies down on the side of the upper stuffing 6 in the mattress device in the state that the bag bodies 19 are initially filled with air. Then, by operating an air pump, air in the bodies 19 are initially filled with air. Then, by operating an air pump, air in the bag bodies 19 is gradually discharged, thereby changing hardness of the upper stuffing 6. When its hardness for the user's linking is attained, operation of the air pump is stopped. By retaining this state, it is used.