PURPOSE: To avoid the effect of noise by reducing number of information transmission lines from an element to the utmost.

CONSTITUTION: An address generating section 4 of a controller 3 generates an address of a unit 2 connected in series with the controller 3 by an optical cable 13. A photoelectric conversion section 6 of the unit 2 converts an optical signal and an address discrimination section 7 discriminates its own address. Moreover, a bit addition section adds an output signal of the element after an address. A series of signals are converted into an optical signal by an electrooptic converter 9 and the signal is sent to the succeeding unit 2 through the optical cable 13. The unit at the final stage sends the optical signal including all the address numbers and the output signal of the element to the controller 3. A photoelectric conversion section 10 of the controller converts the optical signal into an electric signal and a decoding section 11 decodes the output of the element of each unit. Since only two transmission lines, optical cables are employed, no noise effect is caused.