[Problem] To provide a comfortable backrest chair.[Solution] A shape retaining material is formed to have radial cores 12 distributed and radially extending upward from a base plate adjacent a cut hole 4 to be fitted on a chair support frame, and a first and a second reinforcing cores 13 and 14 arranged adjacent the base plate of the radial cores 12. A core sheet is formed by attaching a first, a second and a third mesh sheets 9 to the shape retaining material. A chair sheet material is formed by attaching a first and a second buffer sheets 15 and 16 and an outer surface side buffer sheet 7 to the core sheet. A backrest portion of the chair sheet material is constructed to have a larger right and left width than the breadth of a seated person's shoulders, and when the load of the seated person is applied, right and left opposite ends of the backrest portion can be displaced in directions to approach each other due to elasticity of the core sheet. An upper extending part of the backrest portion is capable of supporting the head of the seated person, and bending to move further rearward than the backrest portion due to the elasticity of the core sheet.