Method of manufacturing a food container or cover includes the steps of: forming a paperboard article from pulp and/or other vegetable fiber material; coating front surface or back surface of the paperboard article with heat sealable thermoplastic resin; forming foldout pieces for connecting adjacent sidewall pieces at four corners of the paperboard article; and forming the food container or cover by hot-pressing the paperboard article thus obtained between a pair of male and female press dies. One mutually opposed pair of the sidewall pieces is configured to be erected before the other mutually opposed pair of the sidewall pieces. The V-shaped foldout pieces include a V-shaped foldout portion shorter than portions that are adjacent to both side portions on the foldout piece, and the V-shaped foldout piece is overlapped or stuck to an adjacent one side of the foldout piece or sidewall piece.