To provide a technology of increasing coding efficiency for an isolated column which appears with a high frequency. Bit map data which indicates an image by an array of pixels of main scanning and subscanning directions is scanned in the main scanning direction, the data is separated into an area including a black pixel and an area including no black pixel by units of a predetermined number of logical lines based on a result of the main scanning, the area including the black pixel which has been separated in the first area separation step is scanned in the subscanning direction, the data is separated into a coded area coincident with a predetermined coded pattern and a noncoded area noncoincident with the coded pattern, an isolated column coincident with a predetermined isolated pattern is separated from the noncoded area separated, and the areas are coded by combining a code indicating the isolated column with a code indicating the number of columns of an area adjacent to the isolated column when the areas separated are coded as elements.