From inputted multilevel image data it is generated, for each pixel, image area information constituted by an image area component indicating whether the pixel exists in a character.cndot.line drawing area or halftone area and an image area component indicating whether the pixel is chromatic or achromatic. The information is encoded by a lossless encoding unit on a block basis and stored, and an amount of code is monitored. When the amount of code exceeds a target amount, the lossless encoding unit performs encoding upon changing image area components, of subsequently input image area components, which coincide with a condition. The codes that have already been stored are temporarily decoded by a lossless code re-encoding unit, and the above image area components are changed. The resultant data is then re-encoded and stored.