To obtain a highly reliable insulating film or the like by providing an optical liquid level sensor as a liquid amount monitoring means for triethylborate in a triethylborate supply apparatus.

An optical detector probe 14 at the top of an optical liquid level sensor 13 is arranged at a specified position near the bottom of the body of a TEB(triethylborate) supply apparatus. A light transmitter/receiver 15 and a detector circuit 16 are connected to the optical liquid level sensor 13. The residual amount of TEB is detected by a signal outputted from the detector circuit 16, and when the residual amount is fewer than a specified value, various kinds of control such as TEB supply stoppage or CVD apparatus operation stoppage are conducted. Tetraethoxysilane(TEOS), which is the raw material compound for silicon, is introduced from a TEOS vaporizer to a reaction chamber. As a result, a boron-doped silica film having remarkably low permittivity and excellent insulating capability can be formed.