The invention relates to a rotor support device of a step motor. A magnet support part is formed using an iron sheet by compression processing technology and before forming a center hole, a groove with a diameter larger than that of the center hole is compressed and formed on the upper surface of the lower surface at the position of the center hole and the burrs generated during perforation are arranged around the groove to ensure the burrs not expose outside the groove without damage on a gasket. Because the magnet support part is formed using the iron sheet by compression processing technology, the production cost is saved, the weight is reduced and the pull-in torque character is improved.