PURPOSE: To improve the arithmetic processing capacity of the whole system by executing data transmission between respective unit computers by optical communication using a light receiving device and a light emitting device.

CONSTITUTION: The light emitting states of respective light emitting elements 6 of light emitting devices 4 in all unit computers 2 are made incident upon the light receiving device 5 of each unit computer 2 through a mirror 3. Thereby, each unit computer 2 can simultaneously confirm addresses to be transmitted and transmitting data displayed by the light emitting states of respective elements 6 of the devices 4 of all other unit computers 2 themselves. In the case of transmitting data from a certain unit computer to a specific unit computer 2, the transmitted address for specifying the unit computer 2 to be transmitted and transmitting data are displayed by the light emitting states of respective elements 6 of the device 4. At the time of receiving the transmitted address to the receiving side unit computer 2 in its light receiving device 4, the receiving side computer 2 receives the light emitting state of the element 6 as transmitting data. Thus, data transmission can be simultaneously executed between two optional unit computers 2.