Image forming apparatus with a fixing device controlled for envelope printing

An image forming apparatus capable of forming an image on an envelope including a first side and a second side opposite from the first side, the first side having a pasted portion where parts of a paper are pasted to each other, the second side not having a pasted portion where parts of a paper are pasted to each other and not having a flap, said image forming apparatus comprising:

• an image forming portion configured to form a toner image on the envelope;
• a fixing device including a fixing member, a heating portion configured to heat said fixing member, and a pressing member contacting said fixing member and forming a nip where the envelope is nipped and fed, wherein said fixing device is configured to fix the toner image on the envelope by heat and pressure;
• a pressure changing mechanism configured to change pressure between said fixing member and said pressing member;
• and
• a controller configured to control said pressure changing mechanism such that the toner image is fixed on the envelope by causing said pressure changing mechanism to change the pressure to a first pressure when an instruction to form the image on the second side of the envelope is inputted and by causing said pressure changing mechanism to change the pressure to a second pressure when an instruction to form the image on the first side of the envelope is inputted, the second pressure being higher than the first pressure.