1. A motor control apparatus that controls rotation of a rotor of an electric motor, which has a plurality of windings to form a plurality of phases and is powered from an electric power source, the motor control apparatus comprising: an encoder that outputs an A-phase signal and a B-phase signal at predetermined angular intervals in response to the rotation of the rotor such that a predetermined phase difference is created between the A-phase signal and the B-phase signal; a learning means for executing an initial position learning process, wherein: at time of executing the initial position learning process, the learning means sequentially changes each current exciting phase among the plurality of phases through one complete cycle at a predetermined time schedule during an initial drive operation after turning on of the electric power source, so that a rotational position of the rotor and its corresponding exciting phase coincide with each other at some timing during the initial drive operation, and thereby the rotor is rotated, and the learning means counts the A-phase signal and the B-phase signal during the rotation of the rotor in the initial drive operation and learns a relationship among a count value of the A-phase signal and the B-phase signal counted by the learning means, a rotational position of the rotor and each exciting one of the plurality of phases at an end of the initial drive operation; the learning means determines whether a result of the learning of the learning means is erroneous based on a pattern of the A-phase signal and the B-phase signal at the end of the initial drive operation; and when the learning means determines that the result of the learning of the learning means is erroneous, the learning means re-executes the initial position learning process by re-executing the initial drive operation; and a control means for driving the rotor, wherein the control means determines a current rotational position of the rotor based on a current count value of the A-phase single and the B-phase signal with reference to the result of the learning of the learning means and sequentially changes each current exciting phase among the plurality of phases based on the current rotational position of the rotor in a normal drive operation after the end of the initial drive operation.