To maintain the color stability and density stability in detecting the reflection density of a test patch formed by toner having all of wave length characteristics, irrespective of reflection method by providing a switch for switching the voltage gain of a non-inverting amplifier corresponding to the wavelength of a light emission element and transmittance of the toner.

In this device, an image formation process controlling circuit 200 is composed of a patch data generating circuit 210, an image density sensor 220, a patch data detecting circuit 230 and a CPU 240. The patch data detecting circuit 230 is a circuit for converting an output signal from the image density sensor 220 to a voltage, and composed of the non-inverting amplifier OP, resistive elements R1 to R6, and diode switches S1 and S2. Then, by selectively connecting the resistive elements R5 and R6 with the resistive element R4 by the diode switches S1 and S2, the voltage gain of the non-inverting amplifier OP is switched corresponding to the light wavelength of the light emission element 221 and transmittance of the toner.