In an air mattress (1) including a plurality of levels of air mattress layers (5, 7) which are vertically stacked, in each of the levels of air mattress layers (5, 7), flexible hollow air cells (43, 23) each extending along the width of the air mattress (1) are arranged in parallel with each other in the longitudinal direction of the air mattress (1), air cells (43, 23) in each of the upper air mattress layer (7) and the lower air mattress layer (5) are divided into a plurality of groups such that the air cells (43, 23) are expanded or contracted in units of the groups by supplying or discharging air to/from these air cells (43, 23).