Provided is a duplexer that can maintain isolation characteristics while being able to independently adjust the input/output ground inductances and the common ground inductance of the double-mode filter. The duplexer includes: a transmitting filter; a receiving filter; a package substrate that includes a first substrate, a die attach layer that is patterned onto the first substrate, a second substrate, and an inner layer located between the first substrate and the second substrate; a double-mode filter that constitutes a portion of the receiving filter; a common ground terminal that makes an input ground and an output ground of the double-mode filter commonly grounded, the common ground terminal constituting a portion of the die attach layer; an input-side ground terminal of the double-mode filter, the input-side ground terminal constituting a portion of the inner layer; and an output-side ground terminal of the double-mode filter, the output-side ground terminal constituting a portion of the inner layer and being formed separately from the input-side ground terminal.