PURPOSE: To enable accurate density discrimination through excluding the effects of nonuniform printing or noise on peak values, by printing a specified pattern read from a character generating circuit on a medium, reading the printed specified pattern by a scanning-type sensor, averaging a reading waveform in a predetermined range to obtain an average, and discriminating a density on the basis of the average.

CONSTITUTION: A medium inserted into an inserter 15 is introduced through an inserter- controlling circuit 14 under the control of a processor 1, and is set to a predetermined printing position. The processor 1 reads a specified pattern from a character generating circuit 7, and output the pattern to a printing head 9 through a printing head controlling circuit 8 to print the pattern for density detection. The processor 1 controls an optical sensor 4 to read the specified pattern thus printed, the output of the pattern thus read is converted into a digital value by an A/D converting circuit 5, and the digital value is stored into a RAM 3. The processor 1 reads a reading waveform of the specified pattern stored in the RAM 3, detects a leading edge position and a trailing edge position based on threasholds, determines a density detection range, and averages the output in the range. Thereafter, the density is discriminated based on the output level in the range.