An image forming apparatus includes a fixing unit having a rotary heating member and a rotary pressure member forming a nip therebetween, a guide wall positioned immediately downstream of the fixing unit and on the side of the rotary heating member with respect to a boundary defined by the transfer medium passing the nip, a guide member positioned on the side of the rotary pressure member with respect to the boundary and face to face with the guide wall, the guide member and the guide wall cooperatively guiding the transfer medium discharged from the fixing unit, and a guide member positioning mechanism for shifting the guide member between a first position where the guide member is moved away from the guide wall and a second position where the guide member is moved closer to the guide wall than at the first position.