PURPOSE: To control the particle diameter of oil mist so as to obtain particle size distribution close to that of oil mist, for example, in a blowby gas of an automobile by removing large diameter oil mist from oil mist obtained by a commercial oil mist generator to move particle size distribution to the small particle diameter side.

CONSTITUTION: Oil mist generated in an oil mist generator is led to a three-way pipe in which an inlet pipe 1 is connected to the middle of an outlet pipe 2 at a right angle and made to communicate therewith like a T-shape, and is caused to flow in the outlet pipe 2 from the opening of the inlet pipe 1, and a part thereof is made to hit the inner wall of the outlet pipe 2 by using inertia of the oil mist, allowing particle size of oil mist flowing out from both the ends of the outlet pipe 2 to be controlled.