PURPOSE: To make it possible to set easily a display gradation range by displaying the mean value and standard deviation of the gradation within a concern range on setting the concern region.

CONSTITUTION: Picture data read out from memory 1 are supplied direct to mixer 24 and displayed on display unit 26 via D/A converter 25. A concern region, set by concern region position setter 3, concern region X-size setter 4 and concern region Y-size setter 5, is mark-displayed in the picture on display unit 26 by supplying the output of mark signal generator 6 to mixer 24. At the same time, data equivalent to the picture in the concern region are extracted by gate circuit 7 and the mean value of the input data and its standard deviation are calculated by mean-value calculation circuit 8 and standard-deviation calculation circuit 13 respectively, converted into number display signals by code converter 22 and character generator 23 and displayed in the display picture. Therefore, the setting of the range of display gradation at the time of setting the concern region is simplified.