A fixing device for thermally fixing a developing agent image to a sheet fed in a sheet feeding direction includes: a tubular member; a heater; a nip member; and a backup member. The nip member made of a metal plate includes: a base portion; a connecting portion; and a flange portion. The base portion is plate-shaped. A nip region for nipping the tubular member between the backup member and the nip member is defined exclusively by the base portion and the backup member. The connecting portion extends from the base portion in a direction away from the backup member. The flange portion extends from the connecting portion in a direction opposite to the sheet feeding direction. The connecting portion and the flange portion define a retaining portion at a position confronting an inner peripheral surface of the tubular member for retaining a lubricant agent therein.