An air mat cell unit has a middle-level cell (25) having an air supply port (25a), first and second lower-level cells (26, 27), and a plurality of pairs of upper-level cells (28, 29, 30). Each of the first and second lower-level cells (26, 27) has a length smaller than the length of the middle-level cell (25) and includes an air supply port (26a). The first and second lower-level cells (26, 27) are attached to a lower surface of the middle-level cell (25) in the longitudinal direction of the middle-level cell (25) and communicate with each other. The pairs of the upper-level cells (28, 29, 30) are attached to an upper surface of the middle-level cell (25) in the longitudinal direction of the middle-level cell (25). Each pair of the upper-level cells (28, 29, 30) extend parallel with each other, communicate with each other, and communicate with the middle-level cell (25). A clearance (G1, G2) is set between a specific pair of the upper-level cells (29) and another pair of the upper-level cells (28, 30) adjacent to the specific pair of the upper-level cells (29). The sum of the length of the upper-level cell (28), the length of the upper-level cell (29), and the length of the upper-level cell (30) of the respective pairs is set smaller than the length of the middle-level cell (25).
