The present invention provides a rotation detection apparatus of a rotational axis connected to an output axis system of a reduction gear to count the rotation of a rotational axis driven rotationally, including a reduction gear, with an integer reduction ratio, whose rotational axis of a driving source is connected to an input axis. The apparatus provides a position signal once every rotation of the output axis, and a rotation angle signal in response to the rotation angle of said input axis is output to correlate said output position signal with one of said rotation angle signals to count the rotation of the rotational axis by these position and rotation angle signals. An accurate rotation can be detected even at one-way high-speed rotation.