A fusing device includes: an endless rotatable fusing belt; a nip forming member that is disposed on an inner circumferential surface of the fusing belt; a pressure roller that is in pressure contact with the nip forming member from an outer side of the fusing belt and forms a fusing nip area between the pressure roller and the fusing belt; and a heat source that is disposed inside the fusing belt and heats the fusing belt. The fusing device further includes a heat-conductive member that is disposed on a lateral side of an outer circumference of the fusing belt. The heat-conductive member extends over a width area in a rotation axis direction of the fusing belt.