PURPOSE: To prevent the permeation of gas having a small molecular or atomic radius through a hollow glass product by coating the inner surface and/or outer surface of the glass product with a carbon (compd.)-based protective layer.

CONSTITUTION: A hollow glass product 2 such as a quartz glass vessel is inserted into a hole pierced in a reactor 1, a heating element 3 is put in the product 2 and electric current is supplied to the element 3 to heat the product 2 until the outer surface attains to 900-1,000°C. Gaseous hydrocarbon diluted with an inert gas is then introduced into the reactor 1 from the gas inlet 4 and a carbon (compd.)-based protective layer 5 of 500-1,000 thickness is formed on the outer surface of the product 2.