The present invention relates to a method for manufacturing an air mat that reduces the number of manufacturing processes by means of rotational molding, while preventing the generation of defective products, to a device for molding an air mat, and to an air mat manufactured and molded by the method and device. In a method for manufacturing an air mat formed in such a manner as to integrally couple a cushion part having a plurality of air bags arranged therein for containing air at the insides thereof with a plate-shaped lower sheet disposed on the lower surface of the cushion part, the method includes the steps of: preparing a lower mold having a plurality of air bag molding recesses having given depths arranged at given intervals thereon, for molding the cushion part of the air mat.