To make an output level of each wavelength equal even when the signal level of a certain wavelength is increased or reduced due to a failure, etc., by controlling a pass center wavelength of a variable gain equalizing filter and loss in a center wavelength so that electric signal levels that correspond to each detected wavelength can be equal.

A light wavelength multiple signal which is divided by an optical demultiplexer 6 is filtered into respective signals of wavelength λ1 to λn by sweeping a wavelength variable filter 7 according to an operation signal that is outputted from a wavelength variable filter control circuit 9. A variable gain filter control circuit 10 controls center wavelength of a variable gain equalizing filter 5 and the loss of the center wavelength so that levels P1 to Pn of electric signals which correspond to each detected wavelength λ1 to λn can be equal. For this, the circuit 10 controls the filter 5 so as to maintain the current state.