A first data communication device for a power over fiber system (1A, 1, 1B), comprising: a power sourcing equipment device including a semiconductor laser that is configured to oscillate with electric power, thereby outputting feed light; and a transmitter and a receiver for data communication with a second data communication device, wherein the first data communication device is connectable to the second data communication device via an optical fiber cable, wherein the first data communication device is capable of controlling low power supply and high power supply that are configured to be performed by the power sourcing equipment device, feed electric power by the high power supply exceeds feed electric power by the low power supply, wherein the first data communication device is configured to start up the second data communication device by the low power supply, by sending out feed light of low power to the second data communication device, and to enable the high power supply after receiving a predetermined signal light from the second data communication device, and to disable the high power supply before receiving the predetermined light, wherein the first data communication device is configured to transmit a predetermined check signal by signal light after starting the low power supply and when receiving a start light from the second data communication device responsive to the low power supply, wherein the first data communication device is configured to receive a response signal transmitted from the second data communication device being a response to the predetermined check signal by the signal light, and wherein the signal light of the response signal is used as the predetermined signal light. a power sourcing equipment device including a semiconductor laser that is configured to oscillate with electric power, thereby outputting feed light; and a transmitter and a receiver for data communication with a second data communication device, wherein the first data communication device is connectable to the second data communication device via an optical fiber cable, wherein the first data communication device is capable of controlling low power supply and high power supply that are configured to be performed by the power sourcing equipment device, feed electric power by the high power supply exceeds feed electric power by the low power supply, wherein the first data communication device is configured to start up the second data communication device by the low power supply, by sending out feed light of low power to the second data communication device, and to enable the high power supply after receiving a predetermined signal light from the second data communication device, and to disable the high power supply before receiving the predetermined light, wherein the first data communication device is configured to transmit a predetermined check signal by signal light after starting the low power supply and when receiving a start light from the second data communication device responsive to the low power supply, wherein the first data communication device is configured to receive a response signal transmitted from the second data communication device being a response to the predetermined check signal by the signal light, and wherein the signal light of the response signal is used as the predetermined signal light.