An image forming apparatus that enables a post-processing apparatus to perform optimum control in accordance with the type (thickness) of sheet. A user sets on an image forming apparatus main unit 10 a thickness of sheet to be conveyed. A CPU circuit section 150 obtains first thickness-determination information used for the sheet conveying speed control in a finisher 500 from the finisher 500. A plurality of candidate sheet thicknesses that can be set, are determined based on the first thickness-determination information obtained by the CPU circuit section 150 and second thickness-determination information used when image-forming process speed control is performed in the image forming apparatus main unit 10 according to sheet thickness.