To provide an oil separator for blow-by gas, which surely separates oil mist of a minute particle size from blow-by gas without increasing pressure loss.

This oil separator is constructed by providing an electric discharge electrode 30 charging oil mist in the blow-by gas by generating corona discharge at an upstream side in a blow-by gas flow direction in a separator body 12 made of insulation material, and also by providing a counter electrode 34 attracting the charged oil mist by Coulomb force and separating the same from the blow-by gas at a downstream side thereof.