First and second stations have first and second memory elements (2, 3) for storing first and second shared data, respectively, and a data transfer system (DT1) includes third and fourth memory elements (CM 12, CM 14) for storing third and fourth shared data, respectively, first and second transfer period determiner (CNT 90, CNT 92), a first transfer element (26a) working to operate in accordance with the first transfer period to have the second shared data stored in the fourth memory element (CM 14) and operate in accordance with the second transfer period to have the first shared data stored in the third memory element (CM 12), a second transfer element (26b) working for transfer of shared data between the fourth memory element (CM 14) and the third memory element (CM 12), and a third transfer element (26c) working to operate in accordance with the second transfer period to have the fourth shared data stored in the second memory element (3) and operate in accordance with the first transfer period to have the third shared data stored in the first memory element (2).