A wheel speed detecting device includes a countermeasure against the entrance of muddy water. An annular recess groove is provided in an outer circumference of an end portion of a hub on a sensor side. A cup-shaped flange portion extends from the sensor supporting member with the flange portion covering the annular recess groove while maintaining a predetermined gap T1 between the flange portion and the outer circumference of the end portion of the hub. Muddy water heads toward a multi-pole magnet ring by running through the gap T1 formed between the flange portion and the outer circumference of the hub. However, the entrance of muddy water can be effectively prevented by making the gap T1 small. Thus, muddy water that enters the gap T1 is trapped in the annular recess groove.