Patent ID: 7894736

Claim:
An image forming apparatus which forms a toner image on an image carrier, transfers the toner image to a long recording medium longer than a regular size, and fixes the toner image transferred to the long recording medium by a fixing device, the fixing device comprising: a fixing member; a pressure member pressure-contacted with the fixing member thereby forming a nip between the fixing member and the pressure member; a first temperature detecting device for detecting a temperature of the fixing member; a second temperature detecting device for detecting a temperature of the pressure member; a heater for heating the fixing member, wherein the heater includes: a main heating member to which electric power is supplied from a main power source: and an auxiliary heating member to which electric power is supplied from an auxiliary power source, wherein when fixing the toner image by causing the long recording medium to pass through the nip formed between the fixing member and the pressure member while heating the fixing member by only the main heating member to which electric power is supplied from the main power source, if the temperature of the fixing member that is detected by the first temperature detecting device is equal to or lower than a fixation temperature threshold at which it is determined that a temperature required for toner image fixation cannot be maintained, the electric power is also supplied from the auxiliary power source to the auxiliary heating member to heat the fixing member: a remaining amount detecting device that detects a remaining amount of electric power supplied from the auxiliary power source; a calculating device that calculates the amount of accumulated heat of the fixing member and the pressure member from the temperature of the fixing member and the pressure member detected by the first and second temperature detecting devices; a recording medium length calculating device that calculates a recording medium length at which fixation of the toner image onto the long recording medium can be assured, from the remaining amount of electric power supplied from the auxiliary power source, which is detected by the remaining amount detecting means, and the amount of accumulated heat; a comparing device that compares the length of the long recording medium that is used in print operation when a print instruction is input, with the recording medium length at which fixation of the toner image at this time point can be assured; and a control device that starts the print operation only when the recording medium length at which fixation of the toner image can be assured is equal to or greater than the length of the long recording medium that is used in the print operation.