PURPOSE: To facilitate the management of a detailed call data by storing call data of a current month, a preceding month and two months before in a magnetic disk and storing further preceding call data in an optical disk.

CONSTITUTION: A magnetic disk 1 stores call data of a current month, a preceding month and two months before and an optical disk 2 receives and stores further preceding call data. That is, a detailed call data is sent from an exchange 3 to a HOST computer 4, in which computing processing is implemented and the result is stored in the magnetic disk 1 and a terminal equipment 5 retrieves the stored call data of a current month. Then retrieval processing is applied to the stored data of a preceding month and then retrieval processing is applied to the stored data of two months before. Then the call data of further preceding months are transferred to the optical disk 2 and the data is retrieved therefrom.