A method for legitimately unlocking a SIM card lock, using an unlocking server connected to a USSD gateway, is for unlocking a SIM card lock set by a telecommunications service provider to a mobile terminal of a subscriber. Upon receiving an unlocking request that is transmitted from the mobile terminal and through the USSD gateway and that includes subscriber data and a USSD code representing activation of the unlocking request, the unlocking server requests the telecommunications service provider to verify, with reference to the subscriber data, whether the subscriber has a right to unlock the SIM card lock. Upon receiving verification from the telecommunications service provider that the subscriber has the right to unlock, the unlocking server transmits an unlocking program and an unlocking password to the mobile terminal such that the mobile terminal may execute the unlocking program and load the unlocking password for unlocking the SIM card lock.