To protect the waist in the most appropriate state in association with the entire back according to one's body build by integrating a waist cover and a back cover and regulating respective air pressure independently.

A waist cover 5 and back cover 6 are integrated and the sealing line 7 is sealed in such a way that respective air pressure of the waist cover 5 and back cover 6 can be regulated independently. Air pressure can be regulated in minute, independently and the waist can be protected throughout the entirety of the back and the waist. Further, by integrating the waist cover 5 and back cover 6, in the back cover 6, an outside back cover 18 and middle back cover 21 can be formed. A small adjustment in the air pressure can be performed relating to the support of one's body in the connecting part between the waist cover 5 and back cover 6 in accordance with the shape of a chair and one's body build. The upper part of the back cover 6 is divided by a sealing line 8 to keep air out, and as a fitting part 9 to the seat is formed and a fitting member 10 such as a magic tape and a string hole 11 are provided, the most appropriate mat can be fixed in accordance with the shape of a seat and one's body build.