PURPOSE: To effectively use the single key type index file by using a task queue searching function for searching a queue having the task with a requested task identification number.

CONSTITUTION: A terminal and menu process 17 generates a task identification number ID as to a requested transaction, i.e., task each time it gathers data for the transaction to be performed for the record of task data stored in a task queue 19. Therefore, even when different processes generate tasks at exactly the same time, the task identification numbers are different. Consequently, a key and a task identification ID are usable for ordinary processing for random access to a single key type index file for efficiently finding the task record having the requested task identification number ID.