A foreign matter detector and a foreign matter detecting method are provided, with which foreign matter detection performance can be improved. The foreign matter detector comprises: a means for irradiating first inspection light and second inspection light to an inspection area such that their incident planes are perpendicular to each other; a measuring means for measuring the spectrum of reflected light, the reflected light being a reflection of the inspection light at the inspection area; and a means for determining, by analyzing the spectrum of the reflected light, whether any foreign matter is intermingled or not.