A fusing roller of an image forming apparatus including an outer metallic pipe, an inner metallic pipe disposed inside the outer metallic pipe, a resistance heating body disposed between the outer and the inner metallic pipes, generating a resistance heat with a power supply, an end cap and a gear cap respectively connected to both ends of the outer metallic pipe and electrically connected to the resistance heating body, an outer insulator interposed between the resistance heating body and the outer metallic pipe to transmit the resistance heat to the outer metallic pipe, and an inner insulator disposed between the resistance heating body and the inner metallic pipe. As a result, the outer insulator has a higher thermal conductivity than that of the inner insulator.