To keep the posture of a body stably even when the body is rested on a cloth-like or net-like support.

A sheet-like body 4 is arranged on the surface of the cloth-like or net-like support 2 stretched between frames 3 of a chair and supports a person seated and suppresses bending of the support 2, and an adjustment pad 5 is attached detachably on a part of the rear face of the sheet-like body 4 for changing partially the distance between the sheet-like body and the support 2. By attaching or detaching the adjustment pad 5 to or from the sheet-like body 4, the slope of the sheet-like body 4 is adjusted.