To provide a data reader capable of surely reading data from dot code of various sizes by appropriately and also adaptively setting an initial parameter for marker detection so as to surely reduce the possibility of detecting many error markers in the initial stage of dot code reading.

A dot code includes plural data dots arranged in accordance with data to be recorded and a marker which is used to decide a reading reference point for the data dots and holds a prescribed relative magnitude correlation to the data dots about prescribed physical characteristics quantity, and plural dot codes are respectively arranged according to a prescribed format. A parameter setting part 3 sets a parameter that is needed to detect a marker in a marker detecting part 4 from a dot code image stored in an image data storing part 2. The parameter is decided on the basis of the relative magnitude correlation between the marker and the data dots so as to first detect the marker without detecting the data dots.