Abstract: An object of the present invention is to manufacture single crystals of high quality on an industrial production scale by preventing impurities from being mixed in single crystals when the single crystals are produced by the solvothermal method. A pressure vessel body 1, in which a supercritical state is maintained, is made of heat resistant alloy, a portion of the pressure vessel body is open, a corrosion-resistant mechanical lining 5 is provided on an inner face of the pressure vessel and on an entire outer circumferential edge of the opening, and the opening is sealed by an airtight mating face formed out of a corrosion-resistant mechanical lining, which is formed on the outer circumferential edge of the opening, and by an airtight mating face of the corrosion-resistant mechanical lining cover 6 on an inner face of the cover 3 through a corrosion-resistant gasket member.