A data collection system in which, in communications between each of terminal units and a data collector through a power line, the data collector subjects a polling signal generated by a polling generating circuit to spread spectrum modulation by a spreading section, and superimposes the spread signal on the power line, while each of the terminal units subjects the incoming spread signal thereto superimposed on the power line to de-spread spectrum demodulation and recognizes it, and superimposes the data subjected to spread spectrum modulation on the power line after a certain period of time different from that in other terminal unit has passed. The data collector collects the data obtained by subjecting spread signals successively coming thereinto each by being superimposed on the power line to de-spread spectrum demodulation according to the polling.