When an ion is introduced into a semiconductor on which a resist is formed, the ion and the resist react with each other to generate a gas (dissociated gas) and a component of the thus-generated dissociated gas is introduced into the semiconductor, which becomes a factor to deteriorate properties of the semiconductor. According to the invention, the dissociated gas to be generated from an organic film is treated. Particularly, the dissociated gas is treated before an ion introduction is performed. As a method of performing such a treatment, the ion introduction is performed by dividing ion introduction processing itself into a plurality of times. The dissociated gas is generated in a maximum quantity just after the ion introduction is started. For this reason, it is possible to decrease an introduction of a component of the dissociated gas into the semiconductor or prevent the component of the dissociated gas from being introduced into the semiconductor, when ion introduction processing is divided into a plurality of times and, in each of the thus-divided ion introduction processing after a second time thereof, the ion is introduced while removing the dissociated gas from a treatment chamber by performing evacuation.