Abstract: A database management system for accessing the same B-tree index by a plurality of transactions. When a transaction is intercepted at the intermediate stage of an index structure modification process executed by an index structure modification execution unit, the logs of the index structure modification operation at rollback is analyzed by an index structure modification operation log analysis unit. Then, an incomplete index structure change process is completed by an uncompleted index structure modification operation completion and control unit, i.e., the roll-forward operation is performed. In this manner, it is possible to provide an efficient access to the B-tree index wherein even if the tree structure modification operation by one transaction is intercepted at its intermediate stage, another transaction is allowed to access the B-tree index thereafter.