sheet is detected at s2 either of the high speed and the low speed modes is selected as in the above manner in the low speed mode the second sheet will be delivered so as to activate the sensor s2 after the passage of time t b measured by the second timer from when the first sheet activates the sensor s2 therefore the sensors on the downstream side will be turned on by the second sheet after the passage of time t b from when the first sheet is detected consequently if the second sheet was not detected after t b it is judged that the sheet has jammed in between fig8 b in the high speed mode the second sheet will be delivered so as to activate the sensor s2 after the passage of time t a measured by the first timer from when the first sheet activates the sensor s2 therefore the sensors on the downstream side will be turned on by the second sheet after the passage of time t a from when the first sheet is detected consequently if the second sheet was not detected after t a it is judged that the sheet has jammed in between fig8 c these relations between the.