A separator unit of an oil mist separator comprises a front unit including a perforated plate and a plurality of holding studs extending rearward from the perforated plate, a rear unit placed behind the perforated plate and including a base plate that is to be intimately put on a rear surface of the perforated plate and has an opening sized to permit insertion of the holding studs of the front unit, a plurality of connecting studs extending rearward from the base plate and an impinge plate held by leading ends of the connecting studs, and a fiber material piece put on the impinge plate, wherein when the front unit is coupled with the rear unit having the holding studs inserted through the opening of the base plate, the fiber material piece is sandwiched between the leading ends of the holding studs and the impinge plate.