PURPOSE: To remove fine dispersion oil of 1W2μ, by a method wherein, after liquid containing oil is caused to pass through a strainer, the fine dispersion oil is coarse-grained, and a denatured process filter which unstabilizes emulsion is located.

CONSTITUTION: Liquid containing oil sucked from a raw water tank 1 is fed to a strainer 2 whereat dusts, fiber and the like are filtered. The strainer 2, whose filtering capacity is adjusted to its purpose by slecting the meshes of a wire net, will do. The strainer 2 causes a pump 3 to be increased in the durability. Then fine dispersion oil is coarse-grained by passing through a filter 4 in a secondary stage, and emulsion is unstabilized. The filter 4 may be formed in a flat plate, but is formed in an accordion system and in a cassette system which is removable type, whereby a filtering capacity per a volume can be increased.