An oil mist separator (100) for an internal combustion engine that separates an oil component in a gas, which is introduced from a crankcase of the internal combustion engine, from the gas, includes a porous filter (150) that separates, from the gas, the oil component in the gas, the porous filter (150) being provided in a passage, through which the gas passes, and being coated with a counteragent for neutralizing an acid substance.