An apparatus and a method are presented for preparing a single crystal ingot of a compound semiconductor material which contains a high vapor pressure component. The apparatus includes: a furnace housing 78 housing a cylindrical hermetic vessel 20 having a ceiling plate section 22A and a bottom plate section 42. External heaters 36, 38 and 40 surrounding the hermetic vessel 20, and a vapor pressure control section which communicates hermetically with the vessel 20. The vapor pressure control section includes: a vapor pressure control tube 98 having a hermetic inner space formed between an inner wall 102 and a coaxial outer wall 100; a communication conduit 96 which hermetically communicates the inner space of the vapor pressure control tube 98 with the inner space of the vessel 20; heat pipes 108, 112 extending along at least one of either the inner wall or the outer wall; control heaters 110, 114 disposed both on the inside of the inner wall and on the outside of the outer wall of the vapor pressure control section 98.