PURPOSE: To make a mixture sent to a reactor to be a mixture from which a BPSG glass layer correctly grows by continuously measuring density of a gas or the mixture and modifying feed conditions of the gas or the mixture, if necessary.

CONSTITUTION: A storage material storage tank 2 storing an initial gaseous mixture containing diborane and silane is connected with an using reactor 3 by a conduit 1. A gas densimeter 10 having a frame 11 is installed to the conduit 1, the frame fixes a vibration tube 14 which is under magnetizing effect via a conductor 15 between an entrance 12 and an exit 13 and a manometer 16 and a thermometer 17 are incorporated into the whole. Information on frequency from a vibration frequency meter 18 is tinged with pressure and temperature, density is calculated, opening degree of a valve 21 in the conduit 1 is modified, if necessary, and/or additional flow of a mixture composed of diborane and an inert gas from an auxiliary tank 23 is controlled by a valve 22.