width of the gap between the fixed window 102 and the movable window 103 was made 1 mm and the size of the gaps between the circumference of the movable window 103 and the isolating ducts 301 was made 2 mm fig4 illustrates the resultant situations with respect to the variation in the film deposition rates in the cases of having carried out the above process of forming said 30 u03bcm thick a si h film twenty five times in fig4 the plots represented by circles indicate the results obtained when the mw pcvd apparatus of the present invention was used and those represented by crosses indicate the results obtained by carrying out the foregoing film forming process with the use of a mw pcvd apparatus having substantially the same in constitution except that it is provided with neither the movable window 103 nor the etching space 105 as a comparison as shown in fig4 in the case of the comparison which is not provided with neither the movable window nor the etching space the film deposition rate decreased as the number of film formation cycles increased and the fixed window a quartz tube was broken in the fourteenth film formation cycle due to the adverse influence of the film deposited on the circumference of the fixed window on the other hand in the case of the mw pcvd apparatus of the present invention the film deposition rate remained substantially constant and neither the movable window nor the fixed window was broken in fig5 there is shown a second embodiment of the mw pcvd apparatus according to the present invention for continuously forming a functional deposited film on a substrate web which comprises a plurality.