ethod using a gas containing an organic titanium compound of formula (I) ##STR4## wherein R is a hydrogen atom, a lower alkyl group, a C8-13 condensed polycyclic hydrocarbonyl group or a silyl group which is substituted with a lower alkyl and/or an aryl, and a reducing gas, d) etching the titanium or titanium compound film on the remaining interlayer insulating film to form a contact plug, and e)