PURPOSE: To provide an image forming device allowing high image quality to be obtained always stably with little fatigue degradation at the time of performing repeated image formation over a long period of time using this device having a built-in photosensitive body with an excellent electrophotographic characteristic and specially with a high sensitive characteristic.

CONSTITUTION: In an image forming device having at least an electrophotographic photosensitive body and means for charge, exposure, development, transfer and cleaning provided on the outer periphery of this photosensitive body, ozone concentration in the device is 0.5ppm or less, the uppermost layer of the photosensitive body is formed of a charge transport layer (CTL), and the transmission coefficient of oxygen gas per unit film thickness of the charge transport layer (CTL) is 1.0×10-7(cc/cm2.S.cmHg) or less.