Oil mist separator

An oil mist separator configured to be arranged in a recirculation passage that recirculates blow-by gas of an internal combustion engine to an intake passage and to separate oil mist from the blow-by gas, the oil mist separator comprising:

• a case that includes a case main body, an inlet for blow-by gas provided on an upstream side of the case main body in a flowing direction of the blow-by gas, and an outlet for blow-by gas provided on a downstream side in the flowing direction of the case main body;
• an oil collecting portion that is accommodated in the case main body and includes electrode plates that are arranged to be opposed to each other with a space in between and filters each of which is arranged between the adjacent electrode plates;
• and
• a gradual change portion provided in the case, the gradual change portion being located in at least one of a space between the inlet and the oil collecting portion in the flowing direction and a space between the oil collecting portion and the outlet in the flowing direction, wherein a cross-sectional flow area of the gradual change portion is gradually decreased as a distance from the oil collecting portion in the flowing direction increases.