Provided is a bandpass filter (10), including: a filter casing (17) defining a rectangular waveguide (11) having a narrow plane (a) and a broad plane (b); a metal plate (12) having a ladder shape and being arranged in a sandwiched manner inside the filter casing (17) at a center of the broad plane (b) so as to be parallel to the narrow plane (a); a dielectric plate (13) positioned in a vicinity of the metal plate (12); a support rod (14) extending in parallel to a longitudinal direction of the rectangular waveguide (11), the support rod (14) being rotatably supported on the filter casing (17) and configured to support the dielectric plate (13); and a first gear (15) fixed at a tip of the support rod (14). The dielectric plate (13) is rotated along with rotation of the first gear (15), to thereby adjust a bandpass frequency.