A fusing device and an image forming apparatus having the same. The fusing device can include a fusing belt, a heat source disposed in an interior of the fusing belt, a supporting member to support at least a portion of an inner peripheral surface of the fusing belt, a press member mounted while opposing the fusing belt to form a fusing nip, and a nip forming part formed with an opening portion to enable heat emitted from the heat source to be transferred to the fusing belt at a position corresponding to the fusing nip. The nip forming part can have at least one gap maintaining part to prevent change of a gap of the opening portion. The nip forming part is provided at the supporting member or a nip forming member mounted between the heat source and the supporting member. Accordingly, a temperature of the fusing belt can rise quickly by directly heating the fusing belt, and deterioration of fusing performance due to deformation of the nip forming member can be prevented.