A posture holding apparatus comprising: a base onto which a substance is placed; a supporting member which is attached onto the base so that the supporting member freely swings centering on a pivot between a standby state where the supporting member does not support the substance and an active state where the supporting member supports the substance in an inclination state, the supporting member being positioned on a side of the base; and a swinging support member (32a,32b) having a returning force for returning the supporting member to the standby state when the supporting member is inclined; wherein the swinging support member (32a,32b) is structured by a swinging member that is supported such that it can swing freely and that swings in conjunction with the supporting member, a biasing member for applying a biasing force to the swinging member in a direction of return of the supporting member to the standby state, a liquid pressure output member for outputting a liquid pressure in response to a weight of the substance placed on the base, the liquid pressure output member being positioned under the base; characterized by a displacement amount adjusting member, with which a point of action of the biasing force applied from the biasing member to the supporting member is moved in response to a liquid pressure outputted from the liquid pressure output member such that a distance from the pivot to the point of action changes, further comprising: a connecting member (36a,36b) that bridges between the swinging member and the supporting member, and synchronizes the swinging member and the supporting member, wherein the point of action is a connection shaft for connecting the connecting member and the swinging member. a base onto which a substance is placed; a supporting member which is attached onto the base so that the supporting member freely swings centering on a pivot between a standby state where the supporting member does not support the substance and an active state where the supporting member supports the substance in an inclination state, the supporting member being positioned on a side of the base; and a swinging support member (32a,32b) having a returning force for returning the supporting member to the standby state when the supporting member is inclined; wherein the swinging support member (32a,32b) is structured by a swinging member that is supported such that it can swing freely and that swings in conjunction with the supporting member, a biasing member for applying a biasing force to the swinging member in a direction of return of the supporting member to the standby state, a liquid pressure output member for outputting a liquid pressure in response to a weight of the substance placed on the base, the liquid pressure output member being positioned under the base; characterized by a displacement amount adjusting member, with which a point of action of the biasing force applied from the biasing member to the supporting member is moved in response to a liquid pressure outputted from the liquid pressure output member such that a distance from the pivot to the point of action changes, further comprising: a connecting member (36a,36b) that bridges between the swinging member and the supporting member, and synchronizes the swinging member and the supporting member, a connecting member (36a,36b) that bridges between the swinging member and the supporting member, and synchronizes the swinging member and the supporting member,