an electromagnetic clutch with improved transmission torque by means of a low - cost method includes an electromagnetic coil , a rotor that is rotary driven by means of an external drive source , an armature attracted to the rotor by means of a magnetic force of the electromagnetic coil , and a hub structure . the electromagnetic clutch is characterized in that a recessed and projected surface having a surface roughness of 10 μm or more and a hardness of 4 gpa or less is formed on the friction surface of the rotor and / or the friction surface of the armature .