An image forming apparatus comprising: storage means configured to store recording sheets; image forming means configured to form an image on a recording sheet fed from the storage means ; first detection means configured to detect the presence or absence of a recording sheet, preceding an interleaf, stored in the storage means ; second detection means configured to detect a remaining amount of recording-sheets in the storage means ; insertion means including an interleaf tray arranged to load the interleaf fed from the interleaf tray between recording sheets conveyed from the image forming means ; and control means configured to control the timing of the start of interleaf feeding by the insertion means , based on a detection result of the second detection means , the control means being arranged to start interleaf feeding from the interleaf tray regardless of the detection of said recording sheet preceding said interleaf by the first detection means , in a case where the recording-sheet remaining amount of the storage means is more than a first threshold, and the control means being arranged to start interleaf feeding from the interleaf tray based on a detection result of the first detection means , in a case where the recording-sheet remaining amount of the storage means is less than the first threshold, wherein the image forming apparatus has a first mode of changing the feeding origin of a recording sheet, according to the detection of the absence of a recording sheet, preceding an interleaf, in the storage means by the first detection means during execution of an image forming job, and a second mode of changing the feeding origin of a recording sheet, according to detection of a recording-sheet remaining amount of the storage means being less than a second threshold during execution of an image forming job, and wherein the second threshold is larger than the first threshold. storage means configured to store recording sheets; image forming means configured to form an image on a recording sheet fed from the storage means ; first detection means configured to detect the presence or absence of a recording sheet, preceding an interleaf, stored in the storage means ; second detection means configured to detect a remaining amount of recording-sheets in the storage means ; insertion means including an interleaf tray arranged to load the interleaf fed from the interleaf tray between recording sheets conveyed from the image forming means ; and control means configured to control the timing of the start of interleaf feeding by the insertion means , based on a detection result of the second detection means , the control means being arranged to start interleaf feeding from the interleaf tray regardless of the detection of said recording sheet preceding said interleaf by the first detection means , in a case where the recording-sheet remaining amount of the storage means is more than a first threshold, and the control means being arranged to start interleaf feeding from the interleaf tray based on a detection result of the first detection means , in a case where the recording-sheet remaining amount of the storage means is less than the first threshold, wherein the image forming apparatus has a first mode of changing the feeding origin of a recording sheet, according to the detection of the absence of a recording sheet, preceding an interleaf, in the storage means by the first detection means during execution of an image forming job, and a second mode of changing the feeding origin of a recording sheet, according to detection of a recording-sheet remaining amount of the storage means being less than a second threshold during execution of an image forming job, and wherein the second threshold is larger than the first threshold. <EOS>