A fixing device includes a fixing belt, a flat heater, a holder and a pressing roller. The fixing belt is rotatable and cylindrical. The flat heater has a holding body and a heating resistor. The holder is configured to hold the flat heater such that a surface in which the heating resistor is embedded comes into contact with an inner circumferential face of the fixing belt via a lubricant. Of a contact area of the flat heater with the inner circumferential face of the fixing belt, in an area outside longitudinal end portion of the holding body, a groove is formed such that the lubricant flows to a center side of the holding body toward a downstream side in a rotational direction of the fixing belt.