To provide a back rest for a chair, the back rest which corrects a sitting posture on a chair by using a back rest for a chair and which enables a correct posture (a posture for acquiring, open chest, extended dorsal muscle, concentration and relaxation) to be obtained by straightening a sitting posture on a chair, as the sitting posture on a chair exerts a useless force to the waist compared with a standing posture, pressurizing the internal organs, collecting stress through the use of the chair for a long time, and causing sickness.

A method of supporting the back with a back rest in a sitting posture on a chair includes: balancing, at a supporting point, a direction of a load from the back and a direction of a force supporting from the back rest, between the dorsal and the lumbar; ensuring to get a correct posture by primarily pressingly supporting the spinal standing muscle in the center of the back with an arcuate projecting face; and adding adjustment of the back rest 29 of this invention having the arcuate projecting face including position of the back rest 29, and an adjustment function of the contact angle thereof to the back.