Provided are an apparatus for processing a substrate and a method for measuring a temperature of the substrate. The apparatus for processing the substrate includes a temperature measurement part and a light-transmitting shield plate. The temperature measurement part includes a light source, a light receiving part configured to receive reflected light reflected by the substrate or the shield plate among the light irradiated from the light source, and a radiant light emitted from the substrate to measure a quantity of the reflected light and an intensity of the radiant light and a temperature calculation part configured to calculate the temperature of the substrate, to which a contamination level of the shield plate is reflected, by using the quantity of the reflected light and the intensity of the radiant light.