In an antenna duplexer including a high band side filter, a low band side filter, and a phase shifter in a low pass configuration inserted between the high band side filter and an antenna port and taking phase matching of both filters, a resonance inductor resonating with a capacitor on the antenna port side is provided to obtain a attenuation characteristic in an optional band in an out-of-pass band of both filters by the resonance. For instance, the maximum attenuation is held at twice of the center frequency fL of a pass band of the low band side filter. The LC resonance circuit configuration can be desirably modified into a configuration such that a resonance capacitor is provided to an inductor in a phase shifter having a high band side filter configuration.