PURPOSE: To speed up a processing at the time of swapping by making initial setting of data set for substitution beforehand prior to a journal swap processing.

CONSTITUTION: When journal acquisition is requested, an electronic computer 10 acquires a journal in a presently used data set 11. When it is detected that the data set is full during journal acquisition by using the data set 11, it is checked whether data set 12 for substitution is already registered or not. When the data set for substitution 12 is registered, the data set for substitution 12 is immediately registered as present use, and journal acquisition is continued. Thereafter, when the data set 11 is allotted as new substitution while the data set 12 is being used as present use, the electronic computer 10 checks the propriety of use of the data use set 11. When it is usable, initial setting of the data set 11 is set, and the data set 11 is registered as a substitution. As journal allotment is executed asynchronously and separately from the main route, the influence on response capability are lightened.