Abstract: Interference between adjacent sections is prevented in an optical wireless communication system, in which a center device is capable of concurrently communicating through lines with a plurality of parent devices each disposed in one of the sections into which a communication area of the system is divided and each of the parent devices communicates with a plurality of child devices disposed in the same section where said parent device is disposed. The sections is grouped into some groups of sections which do not adjoin each other. Optical wireless communications are done only within one group of sections in each communication period. Some techniques for improving the effect of communication are disclosed.