A fixing device for thermally fixing a developer image transferred onto a recording sheet, includes: a flexible fusing member which is flexibly deformable; a heating element; a nip member disposed in such a manner as to contact with a surface of the flexible fusing member and to allow the flexible fusing member to slide along the nip member; a reflecting plate configured to reflect radiant heat from the heating element in a direction toward the nip member; a backup member configured to nip the flexible fusing member with the nip member to thereby form a nip portion for the recording sheet between the flexible fusing member and the backup member; and a stay configured to support both end portions of the nip member. The reflecting plate has at least one flange portion, and the flange portion is held and supported between the nip member and the stay.