To remove burr on the cam shaft surface caused by press-fitting a signal rotor derived from bending a rolled material and constituting an annular body with a predetermined length provided with a recess portion with a predetermined width along a substantially whole length of the inner periphery in the state where an end portion of the cylindrical body is kept leveled with the end surface of a cam shaft for press-fitting.

A signal rotor 1 is formed by bending a rolled material so as to form a collar portion 1b at one end surface of an annular body 1a with its length substantially equal to the thickness of the rear end portion of an intake cam shaft. An outer edge 1c of the annular body 1a is provided with a protrusion 1d. An inner peripheral surface 1f corresponding to the position between the protrusions 1d, 1d is provided with a shallow recess portion 1g along a substantially whole length of the cylindrical body 1a with its circumferential width being smaller than the circumferential distance between protrusions 1d, 1d with respect to the thickness of the cylindrical body 1a. The signal rotor 1 is oriented to press-fit to the rear end portion of the intake cam shaft by facing the protrusion 1d to the fore of the intake can shaft. Burr is generated only in the portion of the inner peripheral surface 1f between recess portions 1g, 1g, which is discontinuously formed at the inner edge at the front side of the cylindrical body 1a.