To provide an image forming device which can always keep image density proper in spite of a rise in the surface temperature of a photosensitive body of a circulation type both-surface recording system image forming device.

On a paper feed conveyance path L1 of a paper feeding conveyance mechanism 21, a temperature sensor 53 which comes into contact with a conveyed recording medium is arranged to detect the temperature of the recording medium P. The surface temperature of the photosensitive body is estimated from the correlation between photosensitive body surface temperature which is previously found by an experiment and the temperature detected by the temperature sensor 53 on the basis of the temperature of the recording medium P detected by the temperature sensor 53. According to the estimated photosensitive body surface temperature, image formation parameters of an image formation part 10, i.e., at least one of the developing bias voltage of a developing unit 13, a voltage applied to an electrostatic charger 12, and the exposure output of a laser device 15 to the photosensitive body 11 is adjusted to always maintain proper image density.