A distortion compensating apparatus (1, 1A, 1B) for compensating for distortion in a waveform of a received light signal, comprising: a plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) which is adapted to compensate for the distortion in the waveform at respective given compensating amounts through digital signal processing, wherein a combination of operating states of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is changed by on/off switching of each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N), and the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) are cascaded, and each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) has an off coefficient pattern adapted to make the each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) switched off and an on coefficient pattern adapted to make the each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) switched on; and wherein the distortion compensating apparatus (1, 1A, 1B) comprises: a controller which is adapted to control the on/off switching and thereby change the combination, based on the distortion in the waveform caused by an optical transmission path through which the received light signal propagates, wherein the controller is adapted to switch the operating state of each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) by selecting the on coefficient pattern when each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is switched on, and selecting the off coefficient pattern when each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is switched off. a plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) which is adapted to compensate for the distortion in the waveform at respective given compensating amounts through digital signal processing, wherein a combination of operating states of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is changed by on/off switching of each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N), and the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) are cascaded, and each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) has an off coefficient pattern adapted to make the each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) switched off and an on coefficient pattern adapted to make the each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) switched on; and wherein the distortion compensating apparatus (1, 1A, 1B) comprises: a controller which is adapted to control the on/off switching and thereby change the combination, based on the distortion in the waveform caused by an optical transmission path through which the received light signal propagates, wherein the controller is adapted to switch the operating state of each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) by selecting the on coefficient pattern when each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is switched on, and selecting the off coefficient pattern when each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is switched off. a controller which is adapted to control the on/off switching and thereby change the combination, based on the distortion in the waveform caused by an optical transmission path through which the received light signal propagates, wherein the controller is adapted to switch the operating state of each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) by selecting the on coefficient pattern when each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is switched on, and selecting the off coefficient pattern when each of the plurality of fixed amount compensating units (2-1 through 2-N, 3-1 through 3-N) is switched off.