A process for forming a pattern film comprises irradiating a specific portion of a sample with a focused ion beam in an atmosphere whereby the vapour of an organic compound is present thereby polymerising or carbonising the organic compound on the specific portion to form a desired pattern film thereon. The organic compound used has a vapour pressure at 300 DEG K between 1 x 10<-><4> and 5 x 10<-><3> Torr.
