An image forming apparatus has a book binding mechanism for performing a book bind operation using front sheets of a plurality of sizes which can bind a bundle of sheet members recorded by said image forming apparatus, and have limitations on the numbers of recorded sheet members to be bound. The image forming apparatus further has output means for outputting sheet number data according to a bundle of sheet members; judge means for judging whether or not the sheet number data output from said output means is larger than a range of the limitation; detect means for, when the judged result from said judge means is affirmative, dividing the sheet number data, and detecting a divided sheet number value which falls within the range of the limitation; and control means for controlling to perform a book bind operation in units of the sheet numbers detected by said detect means.