To provide a waveguide high-pass filter whose dimension in the direction of the tube axis is smaller than that of the conventional waveguide, even if one end is connected to a coaxial transmission system.

One end of a waveguide for a high-pass filter is short-circuited by a short circuit plate, a probe for conversion with a coaxial cable is erected and projected inside a tube on an H plane, and a waveguide transformer for impedance matching with a standard waveguide for transmission is connected to the other end. The waveguide transformer may be a quarter-wave (g) transformer which changes the tube width into the stepped shape, or a tapered waveguide transformer which changes the tube width to a tapered shape.