A fixing device, comprising: a flexible endless fixing member configured to move in a designated direction and heat a toner image so as to melt the toner image; a metal member fixedly provided so as to face an internal circumferential surface of the fixing member with a clearance between the external diameter of the metal member and the internal circumferential surface of the fixing member and configured to heat the fixing member, the metal member being heated by a heating part (25; 50), wherein an amount of the clearance is larger than 0mm and equal to or less than 1mm; and a pressing member configured to press and contact the fixing member so as to form a nip part where a recording medium is conveyed, characterized in that the amount of the clearance is defined by the following relationship: the amount "A" is equal to or less than "B max " and greater than "B ave " ("B max " ≧ "A" > "B ave "), where "A" represents the amount of the clearance at the time of normal temperature between the fixing member and the metal member, "B max " represents a maximum amount of deformation generated in the metal member when heating is started by the heating member (25; 50) from a normal temperature state to a target fixing temperature, and "B ave " represents an amount of stable deformation generated in the metal member when the temperature of the entire metal member becomes even at the time when the recording medium is fed at said target fixing temperature. a flexible endless fixing member configured to move in a designated direction and heat a toner image so as to melt the toner image; a metal member fixedly provided so as to face an internal circumferential surface of the fixing member with a clearance between the external diameter of the metal member and the internal circumferential surface of the fixing member and configured to heat the fixing member, the metal member being heated by a heating part (25; 50), wherein an amount of the clearance is larger than 0mm and equal to or less than 1mm; and a pressing member configured to press and contact the fixing member so as to form a nip part where a recording medium is conveyed, characterized in that the amount of the clearance is defined by the following relationship: the amount "A" is equal to or less than "B max " and greater than "B ave " ("B max " ≧ "A" > "B ave "), where "A" represents the amount of the clearance at the time of normal temperature between the fixing member and the metal member, "B max " represents a maximum amount of deformation generated in the metal member when heating is started by the heating member (25; 50) from a normal temperature state to a target fixing temperature, and "B ave " represents an amount of stable deformation generated in the metal member when the temperature of the entire metal member becomes even at the time when the recording medium is fed at said target fixing temperature. the amount "A" is equal to or less than "B max " and greater than "B ave " ("B max " ≧ "A" > "B ave "), where "A" represents the amount of the clearance at the time of normal temperature between the fixing member and the metal member, "B max " represents a maximum amount of deformation generated in the metal member when heating is started by the heating member (25; 50) from a normal temperature state to a target fixing temperature, and "B ave " represents an amount of stable deformation generated in the metal member when the temperature of the entire metal member becomes even at the time when the recording medium is fed at said target fixing temperature.