Image forming apparatus and control method therefor

An image forming apparatus, comprising:

• a main conveyance passage leading from a cassette, in which a sheet is set, to a discharge position via a print position and an insert position;
• a print portion that, in a print job, feeds and conveys the sheet from the cassette to the main conveyance passage and prints an image on the sheet passing by the print position;
• an inserter that performs an insert process in which the inserter feeds an insert sheet to the main conveyance passage at the insert position and thereby inserts the insert sheet between sheets conveyed one after another along the main conveyance passage,
• wherein
• in the print job involving the insert process, the print portion conveys, while securing an insert interval between a preceding sheet, which is a sheet immediately preceding the insert sheet, and a succeeding sheet, which is a sheet immediately following the insert sheet, the preceding sheet and the succeeding sheet one after another, and
• the print portion automatically changes the insert interval according to a length of the insert sheet in a sheet conveyance direction.