a method and an apparatus for detecting an error in an image scanning device that obtains image data upon scanning a document by using a light of beam radiated from a lamp , including a setting operation of obtaining initial reference values of a light quantity of the lamp and storing the obtained initial reference values in a memory , a storing operation of measuring the light quantity of the lamp during use of an image scanning system , obtaining measured values for comparing with the initial reference values , and storing the measured values in the memory , and a comparing / determining operation of determining whether to check the error to represent the lamp or a contaminant in an optical path by comparing the initial reference values with the measured values .