A barrier film made of a ZrB2 film is formed by use of a coating apparatus provided with plasma generation means including a coaxial resonant cavity and a microwave supply circuit for exciting the coaxial resonant cavity, the coaxial resonant cavity including spaced apart conductors provided around the periphery of a nonmetallic pipe for reactive gas introduction, the coaxial resonant cavity having an inner height equal to an integer multiple of one-half of the exciting wavelength, the plasma generation means being constructed such that a gas injected from one end of the nonmetallic pipe is excited into a plasma state by a microwave when the gas is in a region of the nonmetallic pipe which is not covered with the conductors and such that the gas in the plasma state is discharged from the other end of the nonmetallic pipe.