A method of evaluating the wavelength filter spectrum response is introduced. The increase of the crosstalk level due to the filtering and the relation between the total crosstalk and the spectral efficiency are derived in detail using the Gaussian filter. Since this method can be applied to various kinds of filter spectrum responses, the ultimate spectral efficiencies of filters are compared. In this comparison, the problem of the box-like filter, which has been considered to be desirable, is revealed, and this is improved by cascading the filter spectrum. The requirement on the rejection floor that inheres in the filter is also made clear.