A method for synthesizing a group II-VI compound semiconductor polycrystal, comprising: preparing: a pBN inner vessel composed of a cylindrical crucible having an opening at an uppermost portion and a disc-shaped lid having a diameter almost the same as an outer diameter at the uppermost opening portion of the crucible; and a heat-resistant outer vessel composed of a cylindrical body having an opening at an uppermost portion and a disc-shaped lid having a profile substantially the same as a profile of the uppermost portion of the outer body, the outer vessel having an inner space with a same height as a total height of the inner vessel, wherein the outer vessel is structured such that the lid of the outer vessel can be fixed on the cylindrical body in a semi-airtight state by a screw, putting two or more raw material elements of group II and group VI in the inner body of the inner vessel and putting the inner lid on the uppermost portion of the inner body, putting the inner vessel in the outer body of the outer vessel and fixing the outer lid on the upper most portion of the outer body, placing the outer vessel in a high-pressure furnace having a heating means, evacuating the high-pressure furnace and filling the high-pressure furnace with an inert gas at a predetermined pressure, heating the inner vessel and the outer vessel by the heating means so as to melt and promote a reaction of the raw material elements in the inner vessel, and cooling the reacted raw material elements so as to promote a growth of a polycrystal, wherein the inert gas pressure in the high-pressure furnace before heating is 0.5 MPa to 1 MPa, wherein the outer vessel is made of graphite and a surface of the outer vessel is covered with a coating layer. preparing: a pBN inner vessel composed of a cylindrical crucible having an opening at an uppermost portion and a disc-shaped lid having a diameter almost the same as an outer diameter at the uppermost opening portion of the crucible; and a heat-resistant outer vessel composed of a cylindrical body having an opening at an uppermost portion and a disc-shaped lid having a profile substantially the same as a profile of the uppermost portion of the outer body, the outer vessel having an inner space with a same height as a total height of the inner vessel, wherein the outer vessel is structured such that the lid of the outer vessel can be fixed on the cylindrical body in a semi-airtight state by a screw, a pBN inner vessel composed of a cylindrical crucible having an opening at an uppermost portion and a disc-shaped lid having a diameter almost the same as an outer diameter at the uppermost opening portion of the crucible; and a heat-resistant outer vessel composed of a cylindrical body having an opening at an uppermost portion and a disc-shaped lid having a profile substantially the same as a profile of the uppermost portion of the outer body, the outer vessel having an inner space with a same height as a total height of the inner vessel, wherein the outer vessel is structured such that the lid of the outer vessel can be fixed on the cylindrical body in a semi-airtight state by a screw, putting two or more raw material elements of group II and group VI in the inner body of the inner vessel and putting the inner lid on the uppermost portion of the inner body, putting the inner vessel in the outer body of the outer vessel and fixing the outer lid on the upper most portion of the outer body, placing the outer vessel in a high-pressure furnace having a heating means, evacuating the high-pressure furnace and filling the high-pressure furnace with an inert gas at a predetermined pressure, heating the inner vessel and the outer vessel by the heating means so as to melt and promote a reaction of the raw material elements in the inner vessel, and cooling the reacted raw material elements so as to promote a growth of a polycrystal, wherein the inert gas pressure in the high-pressure furnace before heating is 0.5 MPa to 1 MPa, wherein the outer vessel is made of graphite and a surface of the outer vessel is covered with a coating layer.