Abstract: A division code is added to both ends of a group of pieces of data registered in an item of a data record L and between each pair of adjacent data, so that a piece of code-added data is produced. Thereafter, a character chain pattern (C1,C2) of each pair of adjacent characters C1 and C2 in the code-added data is prepared, an occurrence frequency Fi of each character Ci in each piece of data H is calculated, and character chain information (F1,F2,H,L) corresponding to each character chain pattern (C1,C2) is prepared. Therefore, a index file in which pieces of character chain information prepared for the same item of a plurality of data records are listed is prepared. When a keyword is input by a user, the division code is added to both ends of the keyword to produce a code-added keyword, and a series of keyword character chain patterns is prepared from the code-added keyword in the same manner.

Abstract: A plurality of characters arranged in a text are numbered in order of occurrence for each character type, and an occurrence number table in which one or more occurrence numbers are listed for each character type is prepared in an occurrence number preparing unit. Also, a set of two character types is defined as a character pattern, and a set of two occurrence numbers of two adjacent characters of which types correspond to one character pattern is defined as an occurrence number pattern. One or more occurrence number patterns respectively corresponding to one character pattern are arranged for each character pattern, and a list of occurrence number patterns is prepared as an index file in an index file preparing unit.