A method includes burying a conductive pattern in an insulating film made of SiOH, SiCOH or organic polymer, treating surfaces of the insulating film and the conductive pattern with plasma which includes a hydrocarbon gas as a treatment gas, and forming a diffusion barrier film, which is formed of an SiCH film, an SiCHN film, an SiCHO film or an SiCHON film, over the insulating film and the conductive pattern with performing a plasma CVD by adding an Si-containing gas to the treatment gas while increasing an addition amount gradually or in a step by step manner.