Apparatus for low pressure chemical vapor deposition. The LPCVD apparatus of this invention has a compound source gas flow path which is formed between the inside and outside quartz tubes of the reactor. With the path, the apparatus supplies the compound source gas from the upper section to the lower section of the reactor and lets the source gas be introduced into the deposition reacting space of the reactor while being sufficiently mixed and sufficiently heated and achieves the desired deposition result of uniform quality and thickness of chemical thin layers. The LPCVD apparatus also prevents introduction of oxygen into the reactor when washing the quartz tubes of reactor using N.sub.2 gas, thus to prevent forming of undesirable oxide on the wafers and to minimize the fraction defective of result wafers. In LPCVD apparatus of this invention, the inside and outside quartz tubes of the reactor are easily separated and assembled with respect to the apparatus housing, thus to be easily washed or substituted with new tubes and to be produced in mass production with lower cost.