A signal converter includes: a dielectric substrate; a first conductor layer disposed on one of opposite sides of the dielectric substrate, while including an input section receiving high-frequency signals inputted thereto; a second conductor layer disposed on the other of the opposite sides of the dielectric substrate; and plural first conducting sections penetrating the dielectric substrate for electrically connecting the first and second conductor layers, while forming a waveguide in the inside of the dielectric substrate with the first and second conductor layers. The first conductor layer is disposed on the dielectric substrate without occupying a separator section disposed on the dielectric substrate. The separator section includes first and second sections extend from the input section towards the waveguide. The first and second sections are separated away from each other for gradually increasing their interval in proportion to a distance away from the input section towards the waveguide.