The image forming apparatus including a sheet conveying path which is arranged at the sheet inserting device and which conveys the sheets on which the images are formed at the image forming apparatus main body and the insertion sheet to the post-processing device and a sheet conveying portion which is arranged at the sheet conveying path and which conveys the sheets being capable of accelerating and decelerating is controlled to accelerate sheet conveying speed of the sheets from the image forming apparatus main body from a predetermined conveying speed at the sheet conveying path, to insert the insertion sheet so as to follow a preceding sheet which is accelerated, and to convey the sheets from the image forming apparatus main body and the insertion sheet to the post-processing device after being decelerated to the predetermined conveying speed.