A cushion body for sitting made of an expanded beads molded body, comprising: a base part having a sitting surface formed of a curved surface that is curved concavely to support, from below, the buttocks of a seated person in a seated posture; wherein the expanded beads molded body shows a load of 10 to 150 N with a bending deflection of 20 mm when measured according to an operation described in JIS K 7221-1: 2006, and the expanded beads molded body shows a 25% compressive stress of 0.08 to 7 MPa when measured according to an operation described in JIS K 6767: 1999, wherein the cushion body further comprises: a pair of side supporting portions each having a side supporting surface formed of a curved surface that is curved continuously from the sitting surface, and projecting from sides of the base part such that the side supporting portions can be bent and deformed outward by a load of a seated person applied to the side supporting surfaces so as to support both left and right parts of the buttocks of the seated person in the seated posture, wherein deformable areas are formed under the side supporting portions so that the side supporting portions can bend downward, wherein the sitting surface includes a rear supporting surface configured to support a rear part of the buttocks of the seated person in the seated posture, and wherein the side supporting portions each include an extended part having an auxiliary rear supporting surface formed of a curved surface that is curved continuously from the side supporting surfaces, for supporting the rear part of the buttocks of the seated person in the seated posture together with the rear supporting surface. a base part having a sitting surface formed of a curved surface that is curved concavely to support, from below, the buttocks of a seated person in a seated posture; wherein the expanded beads molded body shows a load of 10 to 150 N with a bending deflection of 20 mm when measured according to an operation described in JIS K 7221-1: 2006, and the expanded beads molded body shows a 25% compressive stress of 0.08 to 7 MPa when measured according to an operation described in JIS K 6767: 1999, wherein the cushion body further comprises: a pair of side supporting portions each having a side supporting surface formed of a curved surface that is curved continuously from the sitting surface, and projecting from sides of the base part such that the side supporting portions can be bent and deformed outward by a load of a seated person applied to the side supporting surfaces so as to support both left and right parts of the buttocks of the seated person in the seated posture, wherein deformable areas are formed under the side supporting portions so that the side supporting portions can bend downward, wherein the sitting surface includes a rear supporting surface configured to support a rear part of the buttocks of the seated person in the seated posture, and wherein the side supporting portions each include an extended part having an auxiliary rear supporting surface formed of a curved surface that is curved continuously from the side supporting surfaces, for supporting the rear part of the buttocks of the seated person in the seated posture together with the rear supporting surface. a pair of side supporting portions each having a side supporting surface formed of a curved surface that is curved continuously from the sitting surface, and projecting from sides of the base part such that the side supporting portions can be bent and deformed outward by a load of a seated person applied to the side supporting surfaces so as to support both left and right parts of the buttocks of the seated person in the seated posture, wherein deformable areas are formed under the side supporting portions so that the side supporting portions can bend downward, wherein the sitting surface includes a rear supporting surface configured to support a rear part of the buttocks of the seated person in the seated posture, and wherein the side supporting portions each include an extended part having an auxiliary rear supporting surface formed of a curved surface that is curved continuously from the side supporting surfaces, for supporting the rear part of the buttocks of the seated person in the seated posture together with the rear supporting surface.