a structured data storage apparatus inputs a given number of structured data items each including respective hierarchical structures of element data items, stores structured data items in first memory, extracts common hierarchical structure of first group of element data items, which is included in and common to structured data items, from the structured data items, based on frequencies of occurrence of first group in structured data items, inputs structured data item to be stored, which includes common hierarchical structure of first group, assigns element IDs corresponding to respective element data items of first group to storage areas allocated to respective element data items of first group in memory location of first memory, the memory location being allocated to structured data item to be stored, stores first group included in structured data item to be stored in respective storage areas in memory location.