The present invention provides an antenna duplexer comprising a package having a plurality of ceramic layers superposed, and a transmitting filter and a receiving filter which are mounted on a surface of the package. A phase-matching strip line is formed on a surface of one ceramic layer. The package has grounding layers formed respectively on surfaces of upper and lower ceramic layers with the phase-matching strip line interposed therebetween. A plurality of via holes are formed around the phase-matching strip line for connecting the two grounding layers to each other.