An optical power supply system (1C, 1D, 1E) and a load, wherein the load is a communication device configured to output a modulated wave with amplitude modulation, and the optical power supply system (1C, 1D, 1E) comprises: a powered device (310C, 310D) comprising: a photoelectric conversion element configured to convert feed light into electric power; a power line configured to transmit the electric power obtained by conversion to the load; a detector configured to detect a magnitude of the electric power being transmitted from the photoelectric conversion element to the load; and a signal outputter (314, 314D) configured to output a detection signal (319, 319D) of the detector, wherein the detection signal (319, 319D) is a signal corresponding to an envelope of the modulated wave that is output by the load; a power sourcing equipment (110C, 110D) comprising: a laser oscillator configured to convert electric power into feed light and transmit the feed light to the powered device (310C, 310D); a signal receiver (114, 114D) configured to receive the detection signal (319, 319D) from the powered device outside of the power sourcing equipment (110C, 110D); and an output controller configured to control output of the laser oscillator based on the received detection signal (319, 319D) so that the electric power to be transmitted to the load changes according to the amplitude of the modulated wave; a first optical fiber (250A, 250E) configured to transmit the feed light; and a signal path configured to transmit the detection signal (319, 319D). the load is a communication device configured to output a modulated wave with amplitude modulation, and the optical power supply system (1C, 1D, 1E) comprises: a powered device (310C, 310D) comprising: a photoelectric conversion element configured to convert feed light into electric power; a power line configured to transmit the electric power obtained by conversion to the load; a detector configured to detect a magnitude of the electric power being transmitted from the photoelectric conversion element to the load; and a signal outputter (314, 314D) configured to output a detection signal (319, 319D) of the detector, wherein the detection signal (319, 319D) is a signal corresponding to an envelope of the modulated wave that is output by the load; a power sourcing equipment (110C, 110D) comprising: a laser oscillator configured to convert electric power into feed light and transmit the feed light to the powered device (310C, 310D); a signal receiver (114, 114D) configured to receive the detection signal (319, 319D) from the powered device outside of the power sourcing equipment (110C, 110D); and an output controller configured to control output of the laser oscillator based on the received detection signal (319, 319D) so that the electric power to be transmitted to the load changes according to the amplitude of the modulated wave; a first optical fiber (250A, 250E) configured to transmit the feed light; and a signal path configured to transmit the detection signal (319, 319D). a powered device (310C, 310D) comprising: a photoelectric conversion element configured to convert feed light into electric power; a power line configured to transmit the electric power obtained by conversion to the load; a detector configured to detect a magnitude of the electric power being transmitted from the photoelectric conversion element to the load; and a signal outputter (314, 314D) configured to output a detection signal (319, 319D) of the detector, wherein the detection signal (319, 319D) is a signal corresponding to an envelope of the modulated wave that is output by the load; a photoelectric conversion element configured to convert feed light into electric power; a power line configured to transmit the electric power obtained by conversion to the load; a detector configured to detect a magnitude of the electric power being transmitted from the photoelectric conversion element to the load; and a signal outputter (314, 314D) configured to output a detection signal (319, 319D) of the detector, wherein the detection signal (319, 319D) is a signal corresponding to an envelope of the modulated wave that is output by the load; a power sourcing equipment (110C, 110D) comprising: a laser oscillator configured to convert electric power into feed light and transmit the feed light to the powered device (310C, 310D); a signal receiver (114, 114D) configured to receive the detection signal (319, 319D) from the powered device outside of the power sourcing equipment (110C, 110D); and an output controller configured to control output of the laser oscillator based on the received detection signal (319, 319D) so that the electric power to be transmitted to the load changes according to the amplitude of the modulated wave; a laser oscillator configured to convert electric power into feed light and transmit the feed light to the powered device (310C, 310D); a signal receiver (114, 114D) configured to receive the detection signal (319, 319D) from the powered device outside of the power sourcing equipment (110C, 110D); and an output controller configured to control output of the laser oscillator based on the received detection signal (319, 319D) so that the electric power to be transmitted to the load changes according to the amplitude of the modulated wave; a first optical fiber (250A, 250E) configured to transmit the feed light; and a signal path configured to transmit the detection signal (319, 319D).