To provide a small seat base, for reducing fatigue of the knees or legs when a person sitting upright and wearing a kimono or on other occasions, light in weight, easy to operate, improving quality and reducing manufacturing costs in the blowing air-cushion seat base, leg-foldable, routinely portable and useable outdoors/indoors.

The seat base 1A includes a mouth 6 which has a protrusive cap and of which the bottom is provided on an air-cushion body 2A formed of a chamber bellow 5 consisting of a flexible plastic or the like, wherein the bottom is initially molded to form a bucklable and upward-facing tray shape 4A and provided with the head of the cap 6A to be flat-topped. The base 1A can be instantly tightly closed through a vent 6a provided on the outside screw-type mouth 6. Molded seat faces 8A and 8B are formed voluntarily enlargeably on a body 2B with the cap operably disposed. By arranging two bodies 2B and 2B in parallel and equipping each top with a tray-shaped molded seat face 8C, a twin-chamber small seat base 1C that is a foldable folded-part type can be formed. By providing pipe legs 14A and 14B on the back of a rigid seat face 8D in different widths, a simple seat base 1D can be formed with crossly foldable legs 14A and 14B by disposing a bending stay 15 between the bottoms.