A concave spherically curved first support portion (23) capable of supporting the seated buttocks (42), and projecting forward from the lower end of the first support portion (23), the lower side of the thigh (43) is A front protrusion (24) that can be supported, and a convex curved lumbar support (28) that is located above the first support (23) and can support the iliac (151) portion of the waist (44). And an upper protrusion (25). On both sides of the base portion of the upper protrusion (25), a first rib portion (30) is formed.着座状態の臀部（４２）を支持可能な凹状の球状曲面形状の第１支持部（２３）と、その第１支持部（２３）の下端から前方に突出し、太腿（４３）の下側を支持可能な前部突出部（２４）と、第１支持部（２３）の上側に位置し、腰部（４４）の腸骨（１５１）部分を支持可能な凸状湾曲形状の腰部支持部（２８）を備えた上部突出部（２５）とを備える。上部突出部（２５）の付け根部の両側には、第１抉り部（３０）を形成する。
