The present invention relates to a multiplexer/demultiplexer with a connection for inputting and/or outputting an optical signal which has signal components of different wavelengths, a carrier plate (8) with at least one wavelength-sensitive element (11), a focussing member (13) with at least two focussing elements (14, 14′) as well as a detector or signal-generator plate (1), on which at least two detectors (4) or signal generators are arranged. To achieve this object, it is proposed according to the invention that both carrier plate (8) and focussing member (13) are connected to the detector or signal-generator plate (1).