To provide a rotational angle detecting device for securing accurate information on the rotational angle without time lag and a detector incorporated bearing assembly having the rotational angle detecting device, a magnet having an anisotropy in a circumferential direction about the rotation axis is mounted on a rotating member and a magnetic sensor having magnetic sensor elements is arranged on a stationary member so as to confront the magnet. An angle calculating unit for measuring an intensity of magnetic field of the magnet from an output of the magnetic sensor elements and for detecting the rotation angle of the rotating member based on the measured value, and a delay time correcting unit for correcting a delay time between detection of the magnetic field of the magnet by the magnetic sensor elements and output of a detected angle from the angle calculating unit are employed.