A motor drive control apparatus comprising: a sensor provided rotatably in synchronization with a motor rotation shaft of an electric motor configured to apply a steering assistance force to a steering shaft; a rotation information detection function unit including a rotational position information detector (46b, 46c) and a rotation information detector configured to detect rotation information relating to a rotational displacement of the electric motor on the basis of the rotational position information detected by the rotational position information detector (46b, 46c); a motor drive control unit configured to drive and control the electric motor on the basis of the rotation information output from the rotation information detection function unit; and a power control unit configured to, when it is determined that an ignition switch is in an OFF state, intermittently supply power from a battery to the rotation information detection function unit at a first interval set in advance during the OFF state of the ignition switch while a motor rotation number of the electric motor is lower than a preset rotation number set in advance and intermittently supply power from the battery at a second interval shorter than the first interval set in advance while the motor rotation number is equal to or higher than the preset rotation number set in advance; characterized in that the sensor is a ring-shaped or disk-shaped magnet including two or more different magnetic poles arranged alternatingly along a circumferential direction; in that the rotational position information detector (46b, 46c) is configured to detect a magnetic flux of the magnet as rotational position information; and in that, when it is determined that the motor rotation number starts to decrease while the power from the battery is intermittently supplied to the rotation information detection function unit at the second interval, the power control unit intermittently supplies the power from the battery to the rotation information detection function unit at a third interval shorter than the first interval set in advance and longer than the second interval until the motor rotation number becomes lower than the preset rotation number during a decreasing period. a sensor provided rotatably in synchronization with a motor rotation shaft of an electric motor configured to apply a steering assistance force to a steering shaft; a rotation information detection function unit including a rotational position information detector (46b, 46c) and a rotation information detector configured to detect rotation information relating to a rotational displacement of the electric motor on the basis of the rotational position information detected by the rotational position information detector (46b, 46c); a motor drive control unit configured to drive and control the electric motor on the basis of the rotation information output from the rotation information detection function unit; and a power control unit configured to, when it is determined that an ignition switch is in an OFF state, intermittently supply power from a battery to the rotation information detection function unit at a first interval set in advance during the OFF state of the ignition switch while a motor rotation number of the electric motor is lower than a preset rotation number set in advance and intermittently supply power from the battery at a second interval shorter than the first interval set in advance while the motor rotation number is equal to or higher than the preset rotation number set in advance; characterized in that the sensor is a ring-shaped or disk-shaped magnet including two or more different magnetic poles arranged alternatingly along a circumferential direction; in that the rotational position information detector (46b, 46c) is configured to detect a magnetic flux of the magnet as rotational position information; and in that, when it is determined that the motor rotation number starts to decrease while the power from the battery is intermittently supplied to the rotation information detection function unit at the second interval, the power control unit intermittently supplies the power from the battery to the rotation information detection function unit at a third interval shorter than the first interval set in advance and longer than the second interval until the motor rotation number becomes lower than the preset rotation number during a decreasing period.