Access destination determination means determines a read destination and/or a write destination of specified data from nodes corresponding to the data and stored in data store destination storage means. Access probability calculation means calculates an access probability based on a history of accesses to the data, and stores the calculated access probability in access probability storage means. When no node corresponding to the specified data is stored in the data store destination storage means, access probability estimation means estimates an access probability of the data based on access probabilities stored in the access probability storage means, and the access destination determination means determines a write destination of the data based on the estimated access probability.