PURPOSE: To obtain a mixed crystal type polycrystalline body having high quality by melting each raw material having high m.p. and low m.p. in a vessel by heating a bottom of a vertically long vessel at a relatively high temp. and a top of the vessel at a relatively low temp., mixing the both raw materials uniformly by the convection of heat, then cooling the mixture while keeping a temp. gradient setting a lower temp. part at the bottom of the vessel and a higher temp. part at the top of the vessel.

CONSTITUTION: A raw material 1 having a high m.p. (e.g. GaAs), a raw material 2 having a low m.p. (e.g. InAs), and a liquid sealant 3 (e.g. B2O3), are charged to a vertically long vessel 4. Then, gaseous N2 is filled at room temp. in the vessel 4 upto a predetermined pressure by opening a valve 19, and both raw materials 1, 2 are melted by energizing electric current through heaters 10, 11, thus a temp. gradient having a higher temp. part at the bottom of the vessel 4 and a lower temp. part at the top of the vessel is formed permitting thus to cause convection of heat. In this stage, both molten raw materials are mixed by being stirred upwards and downwards by the thermal convection. A temp. gradient having a higher temp. part at the top of the vessel and a lower temp. part at the bottom of the vessel is then formed by adjusting the heaters 10, 11. The whole body is cooled while keeping this temp. gradient causing crystallization successively from the bottom of the vessel. By this method, a polycrystalline body contg. no foams and having good quality is obtd.