PURPOSE: To efficiently pyrolitically-decompose a gaseous raw material by providing a conduit wherein one end thereof is made to an introduction port of the gaseous raw material and the outer end is closed and one or more pieces of holes are provided to the side and providing a heating means for the gaseous raw material injected through the holes around this conduit.

CONSTITUTION: A gaseous raw material introduced through a gas introducing port 4 is introduced into a pyrolysis part 1 through a conduit 8 and one part thereof is decomposed in the pyrolysis part 1. Since one end 16 of the conduit 8 is closed, the gas is injected through the holes 18 provided to the side of the conduit 8 and thereafter blown on the high-temp. tantalum heater 9 and tantalum heat shield 3. The nondecomposed gaseous raw material blown on the tantalum heater 9 and the tantalum heat shield 3 is efficiently pyrolitically- decomposed by both high temp. of the heater 9 and the heat shield 3 and the catalytic action of tantalum, and a vapor-depositing substance is produced. The vapor-depositing substance produced in the pyrolysis part 1 is injected as molecular beams through an injection port 5 for the molecular beams.