An image forming apparatus includes a sheet-feeder for feeding a recording sheet from a sheet cassette. Image forming devices are provided for forming a toner image. An image bearer bears a toner image formed by the image forming devices; and a transferring device transfers the toner image on the image bearer to a recording sheet. A sheet amount detector detects the amount of recording sheets in the cassette. A mode controller allows the image forming devices to start their operations before the sheet feeding section conveys a recording sheet when the amount of recording sheets detected by the sheet amount detector exceeds a predetermined reference amount, and allows the image forming devices to start their operations after the sheet-feeding section conveys the recording sheet when the amount of recording sheets detected by the sheet amount detector is smaller than the reference amount.