To provide a tone wheel that is protected from damage upon an exterior impact and effectively reduce manufacturing costs and weight thereof.

The wheel slip detecting system operates an anti-lock brake system by detecting a slip of the wheel. The wheel slip detecting system comprises a tone wheel having a metallic ring body, a perforations and imperforations which are formed at a constant interval on a flat plate of the ring body, and a flange section that is extendedly formed by bending the outer circumferential surface of the ring body, and a detecting section for detecting the perforations and the imperforations and outputting different detected signals in rotation of the tone wheel, and a control section for controlling operability of the anti-lock brake system based on a detected result of the detecting section. The tone wheel is made of a rolled steel plate, and the detecting section is a magnetic field sensor.