A converged system and migration method capable of preventing system performance degradation with respect to data transfer is proposed. A first calculation node continues executing I/O processing in response to an I/O request from an application; and, upon execution of the I/O processing, the first calculation node issues an I/O processing event notice indicating a transfer status of data on which the I/O processing has been executed. When receiving a migration command from a migration management unit, a second calculation node judges whether or not data stored in a first I/O node is data which should be transferred, based on the I/O processing event notice; and then transfers the data, which should be transferred, to a third calculation node.