Provided is a computer system including a first storage subsystem and a second storage subsystem. The first storage subsystem has a first storage unit and a first control unit. The second storage subsystem has a second storage unit and a second control unit. The first control unit records journals, and identifies, when data stored in the first storage unit is to be migrated to the second storage unit, which ones of the recorded journals are related to the data that is to be migrated. The first control unit chooses the identified journals one at a time in reverse chronological order, and judges whether or not the chosen journal corresponds to a data update made at some point between a current time and a time that precedes the current time by a length of a journal retention term set for the second storage subsystem.