The present invention is a medical posture retainer having a mat and a core member. The mat includes an airtight bag and a cushion layer. The airtight bag is a cover of the mat and has a gas barrier property. An exhaust valve for deflation is attached to the bag. The cushion layer is formed of particles in the bag. The core member includes a plate-like support that is attached to a side face of the hag to keep a certain rise height, and a stopper that is attached to the upper face and the lower face of the airtight bag to restrict the rise height. The core member is attached before deflation of the bag so as to impart a shape necessary for receiving a specific part of a human body.