Abstract: A control device for controlling a medium reader having a medium transport roller whose having in a color distinguishable from a color of a medium. The control device has a decision means for deciding a valid data portion depending on the color of the medium transport roller, from data read by the medium reader. The valid data portion can be decided upon the reading of a first line, and the subsequent reading of the second line and so on can be carried in the range of the valid data portion. The control device also has a decision means for deciding a final line of a medium when the color of all the data read becomes identical to the color of the surface of the roller. A medium reader and a memory medium are also disclosed.