A fixing device to fix an unfixed toner image to the sheet includes a fixing belt, a guide plate, a support member and a pressing roller. The fixing belt has endless shape and rotates with a conveyance of the sheet. The guide plate guides the fixing belt from an inside of the fixing belt so as to rotate. The support member supports the fixing belt along a conveyance path of the sheet from the inside. The pressing roller presses the fixing belt on the support member from an outside of the fixing belt. The fixing belt has a belt width wider than a guide width of the guide plate in a rotational axial direction of the fixing belt. Both end edges of the guide plate in the rotational axial direction are formed to have a shape containing a shape not parallel to a rotational direction of the fixing belt.