To provide a cushion body detachably mounted on the surface of a seat of an automobile and an elastic and properly sticky structure with a limited pressure on crews.

A cushion part 20 has an air permeable soft formed body covered airtight with an air-impermeable skin, an open/close valve 30 is provided to change a gas into or discharge it from a space covered with the air- impermeable skin, and a mounting part is provided to fix the cushion part 20 at a required position of the surface of a seat. The air permeability of the foamed body exceeds 10 cm3/cm2/s to below 1,000 cm3/cm2/s.