An image erasing apparatus includes an erasing unit, a reading unit, and a control unit. The ring is a flat sheet of flexible permanent magnet material with an outer periphery and an inner periphery and includes printed material on the ring which can be cut from the ring and affixed to any metal surface. Weighting factor calculation section 106 sequentially updates weighting factors so as to minimize the difference between a combined signal and its reference signal using the weighting factors calculated by weighting factor calculation section 105 as the initial values.