A container of substantially rectangular cross section is formed of synthetic resin and comprises a container body and a lid member to be detachably mounted to the container body. The container body is provided with longitudinal and lateral side walls and a bottom wall and is capable of being molded as a single piece and thereafter foldable into a container. A locking device for locking the container body and the lid member is located on an outer surface of a side wall of the container body and on a corresponding inner surface of a side wall of the lid. The locking member comprises projections extending outwardly from outer surfaces of the lateral side walls of the container body and corresponding recesses extending inwardly from inner surfaces of the lateral side walls of the lid member. A pair of the longitudinal side walls of the lid member are provided with projected portions formed on upper edges thereof and with recessed portions formed on lower edges thereof, wherein each recessed portion formed so as to have an inner shape corresponding to an outer shape of each projected portion. Projecting pieces are formed on the inner surfaces of the lateral side walls of the container body for abutting against and supportably fixing the contents, which have an outer perimeter surface area smaller than a surface area of the bottom wall of the container body.