A low cost method of commercially manufacturing a food container of practical strength from vegetable-fiber material includes the steps of: forming aboard of a predetermined size from pulp and at least one kind of vegetable fiber material other than the pulp; coating the board with thermoplastic resin having a heat sealing property; providing for forming the folding portions for connecting adjacent sidewall pieces at four corners of the board; and hot-pressing the board thus obtained using a pair of press dies to form the food container, the pair of dies being composed of female die and male die, wherein one of the pairs of opposing sidewall pieces is arranged to be erected prior to the other pair of opposing sidewall pieces and four corners are bonded correspondingly each other by a hot press process in order to form the container.