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.