PURPOSE: To reduce the number of connection lines between switches and a digital controller and make an encoding circuit unnecessary, by using transmission and receiving circuits consisting of photoelectric converting circuits in the input device used in a device where the digital controller is used to control machines.

CONSTITUTION: Different voltages are assigned to switches SW1WSW4 by a voltage divider 1, and these assigned voltages are converted in time division by an analog encoder 5 having an analog multiplexer 2. The output of the encoder 5 is transmitted from a transmission circuit 10 and is received by a receiving circuit 12 and is inputted to an analog decoder 28. Parallel digital signals corresponding to switches SW1WSW4 are outputted by discriminators 14W17 of the decoder 28, and digital signals from discriminators 14W17 are converted to a continuous signal by monostable multivibrators 24W27 and are supplied to a controller. Thus, a complicated encoding circuit is not used, and the number of connection lines between switches SW1WSW4 and the digital controller is reduced.