Characteristics of a low-k insulating film grown on a substrate is modulated in the thickness-wise direction, by varying the ratio of high-frequency input and low-frequency input used for inducing plasma in the course of forming the film, to thereby improve the adhesion strength while keeping the dielectric constant at a low level, wherein the high-frequency input and the low-frequency input for inducing plasma are applied from a single electrode, while elevating the level of low-frequency input at least either at the start of formation or at the end of formation of the insulating film, as compared with the input level in residual time zone.