vapor deposition method at atmospheric pressure and with a composition including more than 6 5 of p 2 o 5 and more than 7 0 of b 2 o 3 fig8 is a graph showing a thermal hysterysis of the heat treatment for the oxide film according to this embodiment of the present invention the process of the heat treatment for the oxide film is described with reference to fig7 and 8 a heating boat 2 having the substrates 4 placed thereon is put in a furnace tube 1 during a boat put in period 21 as shown in fig8 and then is subjected to the heat treatment process 23 at a temperature higher than 900 u00b0 c after the heating up period 22 during the heat treatment period 23 the bpsg film shows the viscosity coefficient which decreases with time and becomes flat due to the surface tension after the given time at the heat treatment period the substrates are cooled at a cooling rate of 5 u00b0 c min from 900 u00b0 c to 850 u00b0 c in a nitrogen gas flow of a flowing rate of 20 l min in accordance with the third embodiment the cooling rate is preferably 7 u00b0 c min during the boat.