A curl straightening method in an image apparatus including a fixing apparatus which includes a nip portion formed by a heating unit which heats one face side of a sheet and a pressurizing unit which presses the sheet to the heating unit side and a curl straightening apparatus which is arranged on a conveying path of the sheet from the fixing apparatus to a paper-discharging outlet to straighten a curl of the sheet, the curl straightening method characterised by comprising: a sheet moisture content calculating step of calculating moisture content of the sheet before being supplied to the fixing apparatus; a temperature measuring step of measuring a temperature of the heating unit and the pressurizing unit; a sheet temperature/moisture content calculating step of calculating a temperature and moisture content of the sheet in the respective conveying sections of multiple conveying sections into which are divided a conveying path from the nip portion to the paper-discharging outlet such that a curvature rate of the conveying path in the respective conveying sections is constant; a conveying time calculating step of calculating a conveying time of the sheet in the respective conveying sections; a conveying section strain calculating step of calculating a residual strain of the sheet in the respective conveying sections from the temperature and the moisture content of the sheet in the respective conveying sections that are calculated in the sheet temperature/moisture content calculating step, the conveying time calculated in the conveying time calculating step, and the stress relaxation characteristics of the sheet; a final curl amount calculating step of calculating a final curl amount using the residual strain of the sheet in the respective conveying sections that is calculated in the conveying section strain calculating step; and a curl straightening apparatus controlling step of controlling the curl straightening apparatus based on the final curl amount obtained in the final curl amount calculating step. a sheet moisture content calculating step of calculating moisture content of the sheet before being supplied to the fixing apparatus; a temperature measuring step of measuring a temperature of the heating unit and the pressurizing unit; a sheet temperature/moisture content calculating step of calculating a temperature and moisture content of the sheet in the respective conveying sections of multiple conveying sections into which are divided a conveying path from the nip portion to the paper-discharging outlet such that a curvature rate of the conveying path in the respective conveying sections is constant; a conveying time calculating step of calculating a conveying time of the sheet in the respective conveying sections; a conveying section strain calculating step of calculating a residual strain of the sheet in the respective conveying sections from the temperature and the moisture content of the sheet in the respective conveying sections that are calculated in the sheet temperature/moisture content calculating step, the conveying time calculated in the conveying time calculating step, and the stress relaxation characteristics of the sheet; a final curl amount calculating step of calculating a final curl amount using the residual strain of the sheet in the respective conveying sections that is calculated in the conveying section strain calculating step; and a curl straightening apparatus controlling step of controlling the curl straightening apparatus based on the final curl amount obtained in the final curl amount calculating step.