A receiver optical module that receives a wavelength multiplexed signal is disclosed. The receiver optical module includes an optical de-multiplexer that generates a plurality of signals contained in the wavelength multiplexed signal depending on wavelengths of the signals. The wavelength de-multiplexer has features that the optical de-multiplexer has a plurality of sub-elements stacked to each other, where each of the sub-elements de-multiplexes a portion of the wavelength multiplexed signal. Or, the optical de-multiplexer has a series of wavelength selective filters each extracting signal components having outermost wavelengths from signal components entering therein.