A pressure vessel for growing single crystals by solvothermal method, the pressure vessel comprising: a pressure vessel body made of a heat resistant alloy for maintaining a supercritical state, the pressure body including an opening; a corrosion-resistant mechanical lining composed of a mechanical lining body provided on an inner face of the pressure vessel and a flange portion provided on an entire outer circumferential edge of the opening; a cover for sealing the opening under a condition that the flange portion formed on the outer circumferential edge of the opening is used as an airtight mating face; and an intermediate covering layer (12a, 12b) which has high corrosion-resistant and nitriding-resistant properties and which is formed as an inner layer of the corrosion-resistant mechanical lining on an inner face of the pressure vessel body with a wall thickness in the range from 1 to 30 mm, and which is positioned between the pressure vessel body and the corrosion-resistant mechanical lining; wherein the corrosion-resistant mechanical lining is composed by joining the flange portion formed on the outer circumferential edge of the opening and the mechanical lining body formed on the inner face of the pressure vessel body, by means of a joining portion, characterized in that the flange portion is formed into an integrated body, with an L-shaped cross section, including a portion of the cylindrical shape of the mechanical lining body, and the cylindrical portions are butted and joined to each other at the joining portion, so that the joining portion of the flange portion and the mechanical lining body is located at a position where a ring-shaped member of the flange portion, the ring-shaped member being the part of the L-shaped cross section flange portion formed on the outer circumferential edge of the opening, is not influenced by the heat of the post-heating treatment. a pressure vessel body made of a heat resistant alloy for maintaining a supercritical state, the pressure body including an opening; a corrosion-resistant mechanical lining composed of a mechanical lining body provided on an inner face of the pressure vessel and a flange portion provided on an entire outer circumferential edge of the opening; a cover for sealing the opening under a condition that the flange portion formed on the outer circumferential edge of the opening is used as an airtight mating face; and an intermediate covering layer (12a, 12b) which has high corrosion-resistant and nitriding-resistant properties and which is formed as an inner layer of the corrosion-resistant mechanical lining on an inner face of the pressure vessel body with a wall thickness in the range from 1 to 30 mm, and which is positioned between the pressure vessel body and the corrosion-resistant mechanical lining; wherein the corrosion-resistant mechanical lining is composed by joining the flange portion formed on the outer circumferential edge of the opening and the mechanical lining body formed on the inner face of the pressure vessel body, by means of a joining portion, characterized in that the flange portion is formed into an integrated body, with an L-shaped cross section, including a portion of the cylindrical shape of the mechanical lining body, and the cylindrical portions are butted and joined to each other at the joining portion, so that the joining portion of the flange portion and the mechanical lining body is located at a position where a ring-shaped member of the flange portion, the ring-shaped member being the part of the L-shaped cross section flange portion formed on the outer circumferential edge of the opening, is not influenced by the heat of the post-heating treatment.