PURPOSE:To efficiently enlarge oil particles and to aggregate to carry out oil-water separation by circulating raw water between a circulating treatment vessel provided with cation supply electrodes in the bottom part without using any outside electric power source and an oil sucking vessel provided with adsorbing aggregation material. CONSTITUTION:Raw water is sent from raw water inlet 1 to a raw water tank 2 in which metallic cations eluted from the cation supply electrodes 3 consisting of different kinds of metallic plates enlarge oil drops in the raw water to promote the aggregation. Then, the raw water is sent through a siphon 4 into the circulating treatment vessel 5 in which the oil drops are again enlarged by metallic cations supplied from the cation supply electrodes 3a. And then, the raw water is sent to the oil sucking vessel 9 by a circulating pump 7 and the oil drops are collected by the oil adsorbing aggregation material 13 while ascending in the oil adsorbing aggregation material 13, and the raw water are returned over a weir 11 into the circulation treatment vessel 5 to be circulated. The collected oil drops are automatically moved through an oil sucking-out unit 17 to an oil reservoir 18.