PURPOSE: To compress data and to shorten the time of a communication processing and the recording processing of the data by dividing measurement data into the areas of a predetermined range and obtaining the difference data of respective points and the minimum value of the area for the area under consideration.

CONSTITUTION: An area minimum value detection circuit DMIN compares the data of the respective points belonging to a predetermined specific area and selects the minimum value, difference data calculation circuits DC1-DCn calculate the difference data of the data of the respective points and the selected minimum value and a difference data maximum value detection circuit DMAX selects the maximum value from the calculated difference data of the respective points. A bit selection/data synthesis circuit CDGEN selects the bit of the difference data to be finally used by the bit constitution of the maximum value of the difference data, synthesizes minimum value data, difference data bit constitution data and bit selected respective difference data and generates compressed data. Thus, the time required for the thereafter is shortened.