Heat device and image forming apparatus including the same

A heat device, which heats a developed image and fixes the developed image onto a recording material while nipping and conveying the recording material bearing the developed image by a nip portion, comprising:

• a heat source including at least two heat generation members which are separately electrified;
• a fixing member heated by the heat source;
• a pressure member for forming the nip portion by being brought into contact with the fixing member;
• and
• a temperature detecting unit for detecting a temperature of the heat source, a first heat generation member of the at least two heat generation members having a turn-on ratio adjusted according to a variable controlled based on an integrated value for a difference between a detected value and a target value for every predetermined period of the temperature detecting unit,
• wherein, during continuous conveyance of the recording material, the heat device controls the at least two heat generation members to switch a turn-on duty ratio of the second heat generation member to a first heat generation member in a direction in which a maximum amount of possible heat generation is decreased, when the variable is smaller than a predetermined value, at a predetermined timing.