The signal combining device includes: a plurality of first filters to subject each of a plurality of reception signals to processing with first filter coefficients, the plurality of reception signals being generated by subjecting optical signals to coherent detection; a plurality of second filters to subject outputs of the first filters to processing with second filter coefficients; a combiner to output combined signals acquired by combining outputs of the second filters; and a controller to perform adaptive control for each of the first filter coefficients and each of the second filter coefficients with different step sizes in each other, so that the combined signals are in a predetermined state, based on the reception signals input to the first filters and the combined signals, and to switch an update state of a filter coefficient of each of the first filters, based on magnitudes of the second filter coefficients.