An automatic chest compressor (10) for repeatedly compressing the chest of a patient, is constructed to have a small thickness (H) and light weight so it can be readily carried by an emergency worker. The chest compressor includes a piston support (34) at the top, a pressing part (40) at the bottom that presses towards the chest of the patient, and piston side walls (36) that repeatedly elongate to depress the pressing part. The piston side wall are flexible material that is repeatedly curled and uncurled as the side walls move the pressing part respectively up and down.