A wavelength multiplexer comprises a plurality of variable attenuators (12) for variably attenuating a plurality of optical signals of different wavelengths, an optical multiplexer (14) for combining optical outputs from the attenuators, and an optical amplifier (16) for amplifying the optical output from the optical multiplexer. To avoid the effects of the leakage light of unused wavelength on the optical amplification of wavelength in use, an optical switch (46) is provided before the variable attenuators, and sufficient attenuation of unused wavelength is secured by switching to an attenuator film.