A communication system comprising: a plurality of slave devices (3, 301, 302, 303); and a master device communicating with the plurality of slave devices (3, 301, 302, 303); a plurality of types of circuit blocks (6, 7, 8, 9, 10A, 10B, 11A, 11B), each with different functions and formed on separate substrates, and in that the slave devices (3, 301,302, 303) and the master device are configured by combining the plurality of types of circuit blocks (6, 7, 8, 9, 10A, 10B, 11A, 11 B), and characterised by: a writing device for writing an ID to the slave devices (3, 301, 302, 303, 303) and the master device, wherein after transmitting the ID of the master device, the writing device sequentially transmits the IDs of the plurality of slave devices (3, 301, 302, 303), and in that the master device includes: a plurality of output switches (CH1, CH2, CH3, CH4) provided for each of the plurality of slave devices (3, 301, 302, 303) and for supplying power to the corresponding slave devices (3, 301, 302, 303); a first setting unit that receives the ID of the master device from the writing device and sets it as its own ID; and a switch control unit that turns on the output switch (CH1, CH2, CH3, CH4) in order each time the ID of the plurality of slave devices (3, 301, 302, 303) is received from the writing device after setting its own ID, and the plurality of slave devices (3, 301, 302, 303) has a second setting unit that sets the ID received after power supply as its own ID. a plurality of slave devices (3, 301, 302, 303); and a master device communicating with the plurality of slave devices (3, 301, 302, 303); a plurality of types of circuit blocks (6, 7, 8, 9, 10A, 10B, 11A, 11B), each with different functions and formed on separate substrates, and in that the slave devices (3, 301,302, 303) and the master device are configured by combining the plurality of types of circuit blocks (6, 7, 8, 9, 10A, 10B, 11A, 11 B), and characterised by: a writing device for writing an ID to the slave devices (3, 301, 302, 303, 303) and the master device, wherein after transmitting the ID of the master device, the writing device sequentially transmits the IDs of the plurality of slave devices (3, 301, 302, 303), and in that the master device includes: a plurality of output switches (CH1, CH2, CH3, CH4) provided for each of the plurality of slave devices (3, 301, 302, 303) and for supplying power to the corresponding slave devices (3, 301, 302, 303); a first setting unit that receives the ID of the master device from the writing device and sets it as its own ID; and a switch control unit that turns on the output switch (CH1, CH2, CH3, CH4) in order each time the ID of the plurality of slave devices (3, 301, 302, 303) is received from the writing device after setting its own ID, and the plurality of slave devices (3, 301, 302, 303) has a second setting unit that sets the ID received after power supply as its own ID. a writing device for writing an ID to the slave devices (3, 301, 302, 303, 303) and the master device, wherein after transmitting the ID of the master device, the writing device sequentially transmits the IDs of the plurality of slave devices (3, 301, 302, 303), and in that the master device includes: a plurality of output switches (CH1, CH2, CH3, CH4) provided for each of the plurality of slave devices (3, 301, 302, 303) and for supplying power to the corresponding slave devices (3, 301, 302, 303); a first setting unit that receives the ID of the master device from the writing device and sets it as its own ID; and a switch control unit that turns on the output switch (CH1, CH2, CH3, CH4) in order each time the ID of the plurality of slave devices (3, 301, 302, 303) is received from the writing device after setting its own ID, and the plurality of slave devices (3, 301, 302, 303) has a second setting unit that sets the ID received after power supply as its own ID. a plurality of output switches (CH1, CH2, CH3, CH4) provided for each of the plurality of slave devices (3, 301, 302, 303) and for supplying power to the corresponding slave devices (3, 301, 302, 303); a first setting unit that receives the ID of the master device from the writing device and sets it as its own ID; and a switch control unit that turns on the output switch (CH1, CH2, CH3, CH4) in order each time the ID of the plurality of slave devices (3, 301, 302, 303) is received from the writing device after setting its own ID, and the plurality of slave devices (3, 301, 302, 303) has a second setting unit that sets the ID received after power supply as its own ID.