A voltage generated when a predetermined electric current is applied to a transfer
roller is measured, and a sheet transport speed is determined based on the measured
voltage, a sheet type, and a sheet width. The transport speed is reduced when current
leakage tends to occur due to a narrow sheet width or low resistance in the transfer
roller, so that the electric charge per unit area applied to the sheet is maintained
constant without increasing a transfer bias to be applied to the transfer roller.