a method for encoding image data in conformity with joint bi - level image group system , comprising the steps of : determining whether or not a typical prediction should be performed ; if a result of determination at step is negative , determining whether or not all the pixels in a region composed of lines including pixels constituting a context are white ; if a result of determination at step is affirmative , determining whether or not a predicted value corresponding to a context of which all the pixels are white is white ; if the result of determination at step is affirmative , if the result of determination at step is negative , or if a result of determination at step is negative , performing a first single line encoding process ; and if the result of determination at step is affirmative , performing a second single line encoding process .