A fixing device includes a heating rotating body, a pressure rotating body, and a fixing frame. The fixing frame supports the heating rotating body and the pressure rotating body. The fixing frame includes a pair of side plates and a plurality of stays. The side plates are disposed to face each other, and supports both axial end portions of the heating rotating body and both axial end portions of the pressure rotating body. The stays are disposed between the side plates. In a XYZ coordinate in which an axis along a direction in which the side plates face each other is defined as the X-axis, and axes perpendicular to the X-axis and perpendicular to each other are defined as the Y-axis and the Z-axis, each of the stays is fastened to the side plates at least two axial directions of the three axial directions of the XYZ coordinates.