A restart control system includes a rotational angle estimation unit that estimates a rotating electrical machine's rotational angle in a state wherein an internal-combustion engine's rotational speed detected by a rotational speed detection unit is higher than an estimation threshold and configured not to estimate rotating electrical machine's rotational angle in a state wherein internal-combustion engine's rotational speed is lower than estimation threshold; a rotating electrical machine control unit increases internal-combustion engine's rotational speed by rotating electrical machine under condition where predetermined restart conditions are satisfied and rotating electrical machine's rotational angle is estimated by rotational angle estimation unit, thereby restarting the internal-combustion engine; and a starter motor control unit starts driving of a starter motor under a condition where predetermined restart conditions are satisfied and internal-combustion engine's rotational speed falls below a predetermined rotational speed set lower than estimation threshold, thereby increasing internal-combustion engine's rotational speed to estimation threshold.