A mail sorting system comprising a primary sorting post office for performing primary sorting of mails as arrived mail processing and a secondary sorting post office for performing a secondary sorting of the primary sorted mails, wherein: the primary sorting post office comprises at least ID printing means for printing an ID on each arrived mail, address recognition means for sequentially recognizing address information necessary for primary sorting and secondary sorting from an address image of each mail and for saving the address information in association with the IDs in a database, sorting control means for sorting the mails in sorting boxes corresponding to the secondary sorting post office based on the address information necessary for primary sorting recognized by the address recognition means and for transferring mails in any of the sorting boxes into a transportation/storage medium at the time of filling the sorting box, and information writing means for acquiring from the database the address information necessary for secondary sorting of the mails that are transferred into the transportation/storage medium and for recording the address information in association with the IDs on a recording medium on the transportation/storage medium in such manner to record and read information; and the secondary sorting post office comprises at least information read-out means for reading out the address information for each ID recorded on the recording medium on the transportation/storage medium from the primary sorting post office, and delivery sorting control means for reading out the ID printed on each mail contained in the transportation/storage medium, acquiring the address information for the mail by the address information for each ID that is read out by the information read-out means, and sorting the mails in sorting boxes for delivering the mails based on the acquired address information. the primary sorting post office comprises at least ID printing means for printing an ID on each arrived mail, address recognition means for sequentially recognizing address information necessary for primary sorting and secondary sorting from an address image of each mail and for saving the address information in association with the IDs in a database, sorting control means for sorting the mails in sorting boxes corresponding to the secondary sorting post office based on the address information necessary for primary sorting recognized by the address recognition means and for transferring mails in any of the sorting boxes into a transportation/storage medium at the time of filling the sorting box, and information writing means for acquiring from the database the address information necessary for secondary sorting of the mails that are transferred into the transportation/storage medium and for recording the address information in association with the IDs on a recording medium on the transportation/storage medium in such manner to record and read information; and the secondary sorting post office comprises at least information read-out means for reading out the address information for each ID recorded on the recording medium on the transportation/storage medium from the primary sorting post office, and delivery sorting control means for reading out the ID printed on each mail contained in the transportation/storage medium, acquiring the address information for the mail by the address information for each ID that is read out by the information read-out means, and sorting the mails in sorting boxes for delivering the mails based on the acquired address information.