A carriage motor which is a heat generating device is arranged next to a CIS in a scanner unit. The carriage motor drives respective transport rollers of an ADF unit via a gear system as well. The scanner unit makes a white reference board be read so as to obtain white reference data, and obtains black reference data more frequently than the white reference data, e.g., in a continuous process for reading a document. As a CIS module continues a reading process, the black reference data changes as being affected by a change in temperature in the CIS module, etc. The scanner unit deals with such a change in the black reference data by obtaining the black reference data more frequently.