An improved apparatus for growing a single crystal of semiconductor silicon by the Czochralski method which apparatus has a heat-resistant and heat-insulating covering board in direct contact with the upper ends of heat-insulating members surrounding a quartz glass silicon melting crucible and having a circular center opening, and a heat-resistant and heat-insulating tube having an outer diameter approximately equal to the diameter of the center opening in the covering board gas-tightly joined to an upwardly extending argon gas supplying conduit and extending downwardly from the joint of the top wall of a metal housing and the upwardly extending conduit and extending through the center opening in the covering board in such a manner that the single crystal during growing is coaxially surrounded thereby, the lower end thereof being at a height above and in the proximity of the surface of the melt in the crucible. The wafers cut from the silicon single crystal grown in the apparatus are substantially free of defects such as OISFs after a thermal oxidation treatment.