PURPOSE: To make possible maintenance suitable for the detailed degree of data and to reduce man-hour for maintenance by hierarchizing data structure based on specified features, grouping retrieval keys and setting a hierarchical display flag later.
CONSTITUTION: A data base 1 is provided with a least-significant hierarchy A11 of original data structure, next-order hierarchy B12 of grouping key data structure grouped based on the specified features of the hierarchy A11, and most-significant hierarchy C13 grouping the hierarchy B12 based on the specified features. On the other hand, a data preparing part 2 is provided with retrieving parts 21, 22 and 23 for retrieving data corresponding to the respective hierarchies A11, B12 and C13 and a flag setting part 23 for setting a flag for hierarchy identification to retrieving data. Thus, the high-order hierarchy is retrieved from the low-order hierarchy by key items grouping the retrieval keys. When the data exist, a flag showing the retrieved hierarchy is added to service data, and data service is performed.