An oil mist separator comprising: a housing body through which an oil mist containing gas flows, the housing body including upper and lower members (2, 3) that are coupled to constitute therebetween a gas flow passage (5, 12) for the oil mist containing gas; and a separator unit (10, 100) installed in the gas flow passage, the separator unit comprising: a front unit including a perforated plate (20, 120) and a plurality of holding studs (24, 124) extending in a downstream direction from the perforated plate; a rear unit placed at a downstream side of the perforated plate, the rear unit including a base plate (27, 127) that is to be put on a downstream surface of the perforated plate and has an opening (28, 128) sized to permit insertion of the holding studs of the front unit, a plurality of connecting studs (31, 131) extending in a downstream direction from the base plate and an impinge plate (26, 126) held by leading ends of the connecting studs; and a fiber material piece (35, 135) put on an upstream surface of the impinge plate, wherein when the front unit is coupled with the rear unit having the holding studs passed 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. a housing body through which an oil mist containing gas flows, the housing body including upper and lower members (2, 3) that are coupled to constitute therebetween a gas flow passage (5, 12) for the oil mist containing gas; and a separator unit (10, 100) installed in the gas flow passage, the separator unit comprising: a front unit including a perforated plate (20, 120) and a plurality of holding studs (24, 124) extending in a downstream direction from the perforated plate; a rear unit placed at a downstream side of the perforated plate, the rear unit including a base plate (27, 127) that is to be put on a downstream surface of the perforated plate and has an opening (28, 128) sized to permit insertion of the holding studs of the front unit, a plurality of connecting studs (31, 131) extending in a downstream direction from the base plate and an impinge plate (26, 126) held by leading ends of the connecting studs; and a fiber material piece (35, 135) put on an upstream surface of the impinge plate, a front unit including a perforated plate (20, 120) and a plurality of holding studs (24, 124) extending in a downstream direction from the perforated plate; a rear unit placed at a downstream side of the perforated plate, the rear unit including a base plate (27, 127) that is to be put on a downstream surface of the perforated plate and has an opening (28, 128) sized to permit insertion of the holding studs of the front unit, a plurality of connecting studs (31, 131) extending in a downstream direction from the base plate and an impinge plate (26, 126) held by leading ends of the connecting studs; and a fiber material piece (35, 135) put on an upstream surface of the impinge plate, wherein when the front unit is coupled with the rear unit having the holding studs passed 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.