A fusing device includes a pressure unit, a belt unit to rotate in outer contact with the pressure unit, a nip forming unit to form a nip over a contact portion between the pressure unit and the belt unit, a heating unit to heat the nip forming unit and the belt unit, and a support unit to press and support the nip forming unit constantly and having a plurality of heat transmission portions defined in a parallelogrammic shape of an oblique direction with respect to a traveling direction of the belt unit.