An oil mist detector adapted to be arranged in a crankcase of an internal combustion engine comprising: a casing having an oil mist introduction chamber; and a transparent window provided in the oil mist introduction chamber; a light emitting device and a light receiving device so arranged as to face an outer side of the transparent window provided in the oil mist introduction chamber; wherein the light emitting device is arranged for irradiating light to a predetermined detection region in the oil mist introduction chamber through the transparent window and the light receiving device is arranged for receiving through the transparent window scattering light generated from the irradiated light hitting oil mist present in the detection region, thereby achieving oil mist detection, characterized in that the oil mist detector further comprises a zero point shifting structure (7, 8) provided for making a given amount of background scattering light from the casing incident on the light receiving device under absence of oil mist in the oil mist introduction chamber, and a controller for accepting a detection signal from the light receiving device and performing predetermined calculation processing, wherein the controller comprises a dirt abnormality judgment part which is adapted to receive a zero point signal as a detection signal detecting only background scattering light, and adapted to compare a detection signal value indicated by a detection signal from the light receiving device and a dirt abnormal value being a preset value by which the zero point is allowed to be decreased, and adapted to judge that the transparent window is dirty if the detection signal value is smaller than the dirt abnormal value. a casing having an oil mist introduction chamber; and a transparent window provided in the oil mist introduction chamber; a light emitting device and a light receiving device so arranged as to face an outer side of the transparent window provided in the oil mist introduction chamber; wherein the light emitting device is arranged for irradiating light to a predetermined detection region in the oil mist introduction chamber through the transparent window and the light receiving device is arranged for receiving through the transparent window scattering light generated from the irradiated light hitting oil mist present in the detection region, thereby achieving oil mist detection, characterized in that the oil mist detector further comprises a zero point shifting structure (7, 8) provided for making a given amount of background scattering light from the casing incident on the light receiving device under absence of oil mist in the oil mist introduction chamber, and a controller for accepting a detection signal from the light receiving device and performing predetermined calculation processing, wherein the controller comprises a dirt abnormality judgment part which is adapted to receive a zero point signal as a detection signal detecting only background scattering light, and adapted to compare a detection signal value indicated by a detection signal from the light receiving device and a dirt abnormal value being a preset value by which the zero point is allowed to be decreased, and adapted to judge that the transparent window is dirty if the detection signal value is smaller than the dirt abnormal value. a zero point shifting structure (7, 8) provided for making a given amount of background scattering light from the casing incident on the light receiving device under absence of oil mist in the oil mist introduction chamber, and a controller for accepting a detection signal from the light receiving device and performing predetermined calculation processing, wherein the controller comprises a dirt abnormality judgment part which is adapted to receive a zero point signal as a detection signal detecting only background scattering light, and adapted to compare a detection signal value indicated by a detection signal from the light receiving device and a dirt abnormal value being a preset value by which the zero point is allowed to be decreased, and adapted to judge that the transparent window is dirty if the detection signal value is smaller than the dirt abnormal value.