Abstract: An unlocking system is used with cards each having recorded thereon a secret number, a plurality of subnumbers, a specifying code for selecting at least one of the subnumbers, and an effective period. The system comprises a card reader for reading the data recorded on the card, a memory for storing a secret number, specifying code and effective period, and a processing unit for unlocking. The processing unit reduces the effective period stored in the memory by 1 in terms of unit time, e.g. 1 day, upon lapse of every unit time, e.g. 1 day or 24 hours. The unit further compares the secret number on the card with the secret number in the memory and gives an unlocking instruction when a match is found between the two secret numbers, with the effective period in the memory exceeding a predetermined period, e.g. 0.