A fixing unit includes a heating rotary member for heating a sheet, a pressing rotary member that is pressed against the heating rotary member with a specific pressure, forming a nip in between, and at least one separating member having a far end directed toward a surface of one of the rotary members with a narrow gap (d) on a downstream side of the nip so that the far end of the separating member hooks on and separates the sheet which is going to turn with the one rotary member. The gap (d) between the far end of said separating member and the surface of the one rotary member and a minimum width (L) of a top margin of the sheet along a leading edge thereof where the toner image is not formed are set to satisfy a relationship expressed by the inequality: d+1.5.ltoreq.L.ltoreq.3d+1.5.