A fusing unit which is formed on a printing path of an image forming apparatus and fuses an image to a printing medium, the fusing unit comprising: a heat source; a nip plate which is heated by the heat source, and comprises a nip part to heat and press the printing medium; a reflection member including a heat reflecting surface which is formed on a side of the nip plate and reflects heat generated by the heat source toward the nip part; a belt member which is rotatably provided about the heat source, the nip plate and the reflection member, and guides the movement of the printing medium; and a driving roller, which faces the nip part to form a fusing nip between the nip part and the driving roller corresponding to a width of the nip part, disposed such that the belt member is between the nip plate and the driving roller, and the driving roller rotatably drives the belt member, characterized in that the heat reflecting surface is arranged such that any arbitrary position on the heat reflecting surface satisfies the formula: Φ / 2 − 15 ° ≤ θ ≤ Φ / 2 + 15 °, wherein θ is an acute angle formed at the heat reflecting surface between a line L T and an external portion of a line L 1 and Φ is an obtuse angle formed between the external portion of the line L 1 and a line L 2, wherein L 1 is a straight line extending through a center of the heat source and the arbitrary position on the heat reflecting surface and continuing out of the reflection member so as to define an external portion of the straight line L 1, L 2 is a straight line which crosses the arbitrary position on the heat reflecting surface and is substantially perpendicular to a longitudinal plane which is tangential to a point on an upper surface of the nip part where L 2 meets the nip part, and L T is a line tangent to the reflection member at the arbitrary position on the heat reflecting surface. a heat source; a nip plate which is heated by the heat source, and comprises a nip part to heat and press the printing medium; a reflection member including a heat reflecting surface which is formed on a side of the nip plate and reflects heat generated by the heat source toward the nip part; a belt member which is rotatably provided about the heat source, the nip plate and the reflection member, and guides the movement of the printing medium; and a driving roller, which faces the nip part to form a fusing nip between the nip part and the driving roller corresponding to a width of the nip part, disposed such that the belt member is between the nip plate and the driving roller, and the driving roller rotatably drives the belt member, characterized in that the heat reflecting surface is arranged such that any arbitrary position on the heat reflecting surface satisfies the formula: Φ / 2 − 15 ° ≤ θ ≤ Φ / 2 + 15 °, wherein θ is an acute angle formed at the heat reflecting surface between a line L T and an external portion of a line L 1 and Φ is an obtuse angle formed between the external portion of the line L 1 and a line L 2, wherein L 1 is a straight line extending through a center of the heat source and the arbitrary position on the heat reflecting surface and continuing out of the reflection member so as to define an external portion of the straight line L 1, L 2 is a straight line which crosses the arbitrary position on the heat reflecting surface and is substantially perpendicular to a longitudinal plane which is tangential to a point on an upper surface of the nip part where L 2 meets the nip part, and L T is a line tangent to the reflection member at the arbitrary position on the heat reflecting surface. wherein θ is an acute angle formed at the heat reflecting surface between a line L T and an external portion of a line L 1 and Φ is an obtuse angle formed between the external portion of the line L 1 and a line L 2, wherein L 1 is a straight line extending through a center of the heat source and the arbitrary position on the heat reflecting surface and continuing out of the reflection member so as to define an external portion of the straight line L 1, L 2 is a straight line which crosses the arbitrary position on the heat reflecting surface and is substantially perpendicular to a longitudinal plane which is tangential to a point on an upper surface of the nip part where L 2 meets the nip part, and L T is a line tangent to the reflection member at the arbitrary position on the heat reflecting surface.