A millimeter waveband filter is provided with a resonator formed by a pair of electric wave half mirrors in a transmission line of a waveguide allowing electromagnetic waves in a predetermined frequency range of a millimeter waveband to propagate in a TE10 mode, and allows frequency components centering on the resonance frequency of the resonator to pass therethrough. A high-pass filter which has a transmission line reduced in size so as to have a cutoff frequency matching an upper limit of a lower rejection band of a filter passband is formed in a transmission line between the end of the waveguide and the electric wave half mirror, thereby increasing the attenuation of the lower rejection band.