A fixing unit includes a heating member, a pressing member, a casing, a separating member, and an urging member. The separating member has a separation claw and a rotating unit. The urging member urges the separation claw toward the surface of the heating member. The rotating unit has an engaging portion and guide surfaces opposing at an interval smaller than that of the engaging portion. The supporting portion includes an engagement supporting portion that rotatably engages the engaging portion and an insertion opening. The separating member rotates to retract guide surfaces from a position facing the inner wall surfaces of the insertion opening. The guide surfaces oppose the inner wall surfaces of the insertion opening to bring the engaging portion to the engagement supporting portion and thereafter mounting the heating member on the casing.