PURPOSE: To obtain the image coder easily processing coded data in the unit of words or pages by providing a fill bit insertion number arithmetic means to the coder.

CONSTITUTION: A fill bit number arithmetic means 3 inserts a fill bit so as to attain word packing for each line or page. Then the word packing is executed by increasing/decreasing number of inserted fill bits. A count means 1 confirms whether or not a code length of data of an(EOL+ 1)-th line is a multiple of w word (such as 16 bits) for each line and remaining bits are set to 0 when the code length is less than the 16 bits. Then fill bits are added before an EOL code is generated to insert the fill bits so that the code length of fill bits+EOL is 16 bits. Thus, coded data are easily processed in the unit of words and pages.