An image fixing device includes a heat source configured to generate heat, a fixing belt heated by the heat source and disposed to be rotatable, a rotational member disposed to be opposite to the fixing belt and to press a printing medium to the fixing belt, and a supporting member to support an inner circumferential surface of the fixing belt so as to form a fixing nip between the fixing belt and the rotational member and having a plurality of heat insulation grooves formed in a surface thereof so as to reduce thermal conductivity of the supporting member. By the configuration of the present general inventive concept, it is possible to prevent or minimize a heat loss of the fixing belt and thus to enhance image forming efficiency.