A dielectric waveguide filter includes a dielectric plate including first and second principal surfaces facing each other and a side surface connecting outer edges of the first and second principal surfaces, a first surface conductor at the first principal surface, a second surface conductor at the second principal surface, a side conductor film inside the dielectric plate and connecting the first and second surface conductors, and an internal conductor extending in a perpendicular direction to the first principal surface and electrically connected to neither the first surface conductor nor the second surface conductor. Dielectric waveguide resonant spaces are surrounded by the first surface conductor, the second surface conductor, and the side conductor film.