A high-frequency circuit device includes a dielectric substrate. A planar conductor
is provided on each of top and bottom surfaces of the dielectric substrate and
a slot line is formed on the top surface. Also, undesired-wave propagation preventing
circuits, each including multistage band-elimination filters, are provided on the
top surface of the dielectric substrate, with the slot line therebetween. Each
of the band-elimination filters includes two conductive lines and a resonator which
is provided at a portion of one of the conductive lines and which includes two
spiral lines. Accordingly, propagation of an undesired wave of the band whose center
is the resonance frequency of the resonator can be prevented.
