The present invention provides a coplanar line filter or a duplexer, comprising: a dielectric substrate; a plurality of .lambda./4 coplanar resonators provided on said dielectric substrate, said plurality of .lambda./4 coplanar resonators comprising; a first center conductor having electrical length corresponding to a quarter wavelength; and a ground conductor provided with a gap from said first center conductor; a capacitive coupling portion comprising ai gap provided between said first center conductors of a pair of said .lambda./4 coplanar resonators; and a inductive coupling portion, comprising a guide conductor which electrically connects said first center conductor and ground, provided at a joint portion of a pair of said .lambda./4 coplanar resonators; said plurality of .lambda./4 coplanar resonators being connected in series with said capacitive coupling portion and said inductive coupling portion provided alternately. By the above structure and arrangement, a small-scale coplanar line filter or duplexer of simple design is obtained.