PURPOSE: To obtain the titled device whose handling operation can be executed easily by providing an optical transmissivity detecting device for opening and closing freely a light source case and a photodetecting sensor case, on the outside of a copying machine body, and connecting it to the copying machine body by a flexible cable.

CONSTITUTION: When a correct place for detecting and measuring an optical transmissivity of an original drawing paper is inserted between a light source case 32 and a photodetecting case 33, a start switch 38 is started, and an automatic exposure quantity adjusting device 41 starts operation. Also, when a light source lamp 21 is turned on, a photodetecting sensor 22 detects a transmitting light quantity of an original drawing paper, and an origianl drawing paper optical transmissivity detecting and measuring flexibility 23 of the automatic exposure quantity adjusting device 41 detects and measures its optical transmissivity. This measured voltage is amplified by an amplifying circuit 24, and thereafter, the voltage value is held by a holding circuit 42. This holding voltage is sent to a speed control generating circuit 26 and a timing circuit 43, and the circuit 26 converts it to a speed control signal of a driving motor 28, and adjusts automatically a rotational speed of the driving motor 28 through a motor speed controlling circuit 27.