In an image forming apparatus which adjusts transferring condition according to an output signal from a moisture sensor, a control circuit has an off-time estimation device for estimating a power source interruption period from a surface temperature of a fixing roller and heat radiation elapsed time characteristics of the fixing roller and a time measuring device for measuring the time elapsed after the loading of a sheet feeding cassette. The control circuit further includes a moisture content converter for converting a total elapsed time after loading the sheet feeding cassette and for converting humidity into moisture content data of the transferring sheets. The control circuit controls the output from a high voltage power source of the apparatus, based on the moisture content data.