Disclosed herein is an airtight container. The fastening structure comprises a body with a rim integrally formed at a periphery of an opened upper end of the body, a lid with a channel portion formed at a periphery of the lid to be fitted into the body and maintain an airtight seal between the body and the lid, one or more foldable fastening ribs, each being protruded from the lid in a lateral direction as an integral part of the lid and having a fastening slit perforated through the fastening rib in a lengthwise direction, and one or more fastening pieces corresponding to the fastening ribs, each being protruded from the body in the lateral direction and inserted onto the corresponding fastening slit of the fastening rib, wherein each of the fastening ribs is formed with one or more reinforcing rib-receiving portions of a semi-circular shape, each of the reinforcing rib-receiving portions opened toward a central lower portion of the fastening slit in a lengthwise direction through the fastening rib, and wherein each of the fastening pieces is a protruded piece protruded in a lateral direction at a position corresponding to the fastening slit of the fastening rib, and is integrally formed with a reinforcing rib being a thick portion downwardly protruded from the fastening piece while being extended in a transverse direction of the fastening piece.