A rotation detecting device, which is compatible regardless of whether a process control device for processing an output signal of the rotation detecting device is standard one or new one having high input signal resolving power, and, also, a rotation detecting device equipped bearing assembly having such rotation detecting device, are provided. The rotation detecting device includes an encoder provided rotatably and having a plurality of circumferentially equidistantly arranged to-be-detected poles, and a sensor for detecting the to-be-detected poles. The use is made of a multiplying unit for multiplying the phase of the to-be-detected poles from an output of the sensor. The use is also made of a pulse outputting unit for receiving an output of the multiplying unit or both the output of the multiplying unit and an output of the sensor and output pulses having two multiplying powers different from each other.