To read out the rule information contained in the image information of a document with small memory capacity by registering an underline in a character data constituting a character line in units of that character data based on the positions of the character line and the underline.

A first line is formed (S51) and the head of an ascending order table is pointed (S52). A decision is then made whether or not the pointed position exceeds the end of the ascending order table (S53). When the end is exceeded, the remaining line is formed between the last line already formed and the lower blank (S69). Subsequently, an underline data region between the upper and lower blanks is checked for the underline and when no underline is present, an underline is registered in a character data in units of that character data based on the positions of the character line and the underline before setting an underline in a sentence (S70). According to the method, rule information contained in the document can be read out accurately with small memory capacity.