A cushion for a chair comprising a seat member and a back-supporting member, wherein the seat member is to abut on buttocks of a user seated and comprises two ischium-facing portions, a surface of each ischium-facing portion being to abut on an ischium of the user seated, each ischium-facing portion having a three-dimensional curved surface such that the surface of the ischium-facing portion is concavely curved in a vertical cross section along a left-right direction and is sloped upward toward a rear side in a vertical cross section along a front-back direction, and the back-supporting member is to abut on a back of the user seated and comprises an ilium upper border-facing portion and a lower ribs-facing portion, a surface of the ilium upper border-facing portion being to abut on an ilium of the user seated, a surface of the lower ribs-facing portion being to abut on lower ribs of the user seated, and the surface of the ilium upper border-facing portion being entirely concavely curved in the transverse cross section along the left-right direction, characterized in that the surface of the lower ribs-facing portion being entirely concavely curved in the transverse cross section along the left-right direction, and a portion of the back-supporting member upper than the lower ribs-facing portion having a three-dimensional curved surface such that the surface of the portion of the back-supporting member upper than the lower ribs-facing portion is entirely concavely curved in the transverse cross section along the left-right direction and is inclined rearward toward the upper part in a vertical cross section along an up-down direction, characterized in that a radius of curvature of the three-dimensional curved surface of the upper portion of the back-supporting member along the left-right direction is larger than a radius of curvature of the surface of the lower ribs-facing portion along the left-right direction. a surface of each ischium-facing portion being to abut on an ischium of the user seated, each ischium-facing portion having a three-dimensional curved surface such that the surface of the ischium-facing portion is concavely curved in a vertical cross section along a left-right direction and is sloped upward toward a rear side in a vertical cross section along a front-back direction, and a surface of the ilium upper border-facing portion being to abut on an ilium of the user seated, a surface of the lower ribs-facing portion being to abut on lower ribs of the user seated, and the surface of the ilium upper border-facing portion being entirely concavely curved in the transverse cross section along the left-right direction, the surface of the lower ribs-facing portion being entirely concavely curved in the transverse cross section along the left-right direction, and a portion of the back-supporting member upper than the lower ribs-facing portion having a three-dimensional curved surface such that the surface of the portion of the back-supporting member upper than the lower ribs-facing portion is entirely concavely curved in the transverse cross section along the left-right direction and is inclined rearward toward the upper part in a vertical cross section along an up-down direction, characterized in that a radius of curvature of the three-dimensional curved surface of the upper portion of the back-supporting member along the left-right direction is larger than a radius of curvature of the surface of the lower ribs-facing portion along the left-right direction.