PURPOSE: To attain an exposure control device which is capable of varying linearly a copying density against its operating distance, and improving remarkably its operability by using a variable resistor provided with a centertap, as a variable resistor for setting an exposure.

CONSTITUTION: As for a reference voltage generating circuit 35, resistances 41W 44 are connected in series between a DC power source DC and the ground. A variable resistor 45 is connected in parallel to the resistance 41, and a variable resistor 46 provided with a centertap is connected in parallel to a series circuit of the resistances 42, 43. A centertap 46a of the variable resistor 46 is connected to a connected point of the resistance 42 and 43. Also, voltage obtained by dividing the voltage of the power source DC is fetched as reference voltage from a sliding piece 46b of the variable resistor 46, and it is supplied to a differential amplifier 36. The variable resistor 46 is a variable resistor for setting an exposure, and by constituting its circuit as shown in the figure, a copying density can be varied linearly against an operating distance of the variable resistor 46 for setting an exposure.