Abstract: The present invention has an object to provide a compact quadrature modulator manufactured at low cost in such a manner that an input level is low over a wide high frequency band, and few input signals are wrapped around an output terminal. The quadrature modulator is mainly constructed of a multi-layer substrate into which a 90-degree phase shifter is assembled, a semiconductor integrated modulator circuit into which two sets of 2-phase modulators and an output signal in-phase synthesizing circuit, and a lid. The multi-layer substrate is constructed of "n" pieces of layers in total, namely from a first substrate corresponding to an uppermost layer to an n-th substrate corresponding to a lowermost layer. The first substrate layer is a layer for assembling a semiconductor integrated circuit, or a layer for connecting a substrate with a semiconductor integrated circuit. The 90-degree phase shifter is constituted by distributed coupling circuits having wavelengths of .lambda.