PURPOSE:To surely grasp the cause of system stoppage by clarifying history that a buffer unreturned to a buffer pool is secured and the task which had been used. CONSTITUTION:When a system operates, the task 3 acquires a buffer area 6 from the buffer pool, if necessary. For acquiring a buffer, a buffer control routine 4 is called, and the identifier of the acquired task and that of the task in use are set in the buffer the moment the buffer is acquired. The buffer 6 is handed over to another task according to circumstances. Even a task side receiving the buffer area 6 calls the routine 4, and rewrites the identifier of the task is use by the buffer reception processing of the routine 4. If the buffer 6 becomes unnecessary, the buffer return processing of the routine 4 is called to return the buffer to the pool, and the identifier of the returned task is set in the buffer area 6.