PURPOSE: To compress highly a data and to attain the simplicity of a processing circuit by dividing a line drawing picture element into blocks, discriminating whether all picture elements in each block are identical or mixed with binary value and including this discriminating signal.

CONSTITUTION: The line drawing picture element is grouped into the m×n-set of blocks B in a main scanning and a sub-scanning direction so as to classify the state in the block into three states as (all 0), (all 1) and (mixture of 1 and 0). In case of the (all 0) and (all 1), the data is compressed by providing the number of blocks where its state is consecutive in the unit transmission length of a specific bit number. When the state of mixture of the identical bit pattern is consecutive with the case of (mixture of 0, 1), the data is compressed by using two words and forming the data corresponding to the number of consecutive blocks together with the bit pattern data.