An image forming apparatus comprising: storage means (220, 230) configured to store recording sheets; image forming means configured to form an image on a recording sheet fed from the storage means (220, 230); first detection means (226, 236) configured to detect the presence or absence of a recording sheet, preceding an interleaf, stored in the storage means (220, 230); second detection means (224, 234) configured to detect a remaining amount of recording-sheets in the storage means (220, 230); 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 (224, 234), 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 (226, 236), in a case where the recording-sheet remaining amount of the storage means (220, 230) 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 (226, 236), in a case where the recording-sheet remaining amount of the storage means (220, 230) 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 (220, 230) by the first detection means (226, 236) 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 (220, 230) configured to store recording sheets; image forming means configured to form an image on a recording sheet fed from the storage means (220, 230); first detection means (226, 236) configured to detect the presence or absence of a recording sheet, preceding an interleaf, stored in the storage means (220, 230); second detection means (224, 234) configured to detect a remaining amount of recording-sheets in the storage means (220, 230); 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 (224, 234), 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 (226, 236), in a case where the recording-sheet remaining amount of the storage means (220, 230) 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 (226, 236), in a case where the recording-sheet remaining amount of the storage means (220, 230) 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 (220, 230) by the first detection means (226, 236) 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.