A fixing device for thermally fixing a developing agent image to a sheet includes: a flexible tubular member having an inner peripheral surface and circularly moving in a moving direction while the developing agent image is thermally fixed; a heater; a nip member being in sliding contact with the tubular member; a backup member nipping the tubular member in cooperation with the nip member; a stay covering the heater and supporting the nip member; an electric component positioned opposite to the heater with respect to the stay; and a frame supporting the electric component. The frame includes: an isolating portion interposed between the tubular member and the electric component for isolating the electric component from the tubular member; and a guide portion guiding the inner peripheral surface of the tubular member while the flexible tubular member is moving in the moving direction.