An image forming apparatus has a both-side image forming unit for forming images on both sides of a recording material; a rotatable belt for heating a toner image on a recording material through a belt; a rotatable supporting member, having an elastic layer, for supporting the belt; a rotatable pressing member, pressed toward the supporting member with the belt interposed therebetween, for forming the nip for nipping and feeding the recording material; the pressing member has a surface hardness which is larger than a surface hardness of the rotatable supporting member, wherein a surface hardness of the rotatable supporting member through the belt is different from a surface hardness of the pressing rotatable member by +4 degrees to -8 degrees.