To provide a plastic container such as a thin-walled plastic container whose corner portions and plane rim portions are reinforced so that locking force is strong and there is no distortion or deformation of the container, thereby it is easy to operate locking and separating.

The plastic container is so constituted that an L-shaped rim is provided, as a corner part of the container body, being bent downward from the plane peripheral part, having an edge 6 at its lowest end, and for its lid body, a corner part is provided in communication from the peripheral plane part to a step downward, and an L-shaped plane peripheral part having an L-shaped under cut part 10 being recessed deep inside is provided, and thus the lid body is locked with the container body in such a way that this L-shaped plane peripheral part covers the whole of the L-shaped rim of the container body and, simultaneously, the edge 6 of the container body is locked onto a peripheral groove 10a located on the under cut part 10 of the lid body.