PURPOSE: To retrieve a retrieval item, where characters in plural classifications exist together, without distinction of respective characters by not only converting input data to characters in one classification but also dividing it by a character string in the same classification to generate data for restoration at the time of registering input data and converting a retrieval character string to characters of one form used for registration.

CONSTITUTION: Input data (a) is inputted to a character conversion part 11; and if input data has two-byte characters which can be converted to one-byte characters, a converted character string (b) where they are substituted and data for restoration (c) indicating execution of this conversion are generated and are stored in a storage device 12 correspondingly to each other. Meanwhile, a retrieval character string (d) is inputted to a character conversion part 13 to generate a corresponding converted character string (e) in the same manner. A character comparison processing part 14 compares the converted character string (b) of input data (a) with the converted character string (e) of the retrieval character string (d) and outputs coincidence/disaccord information (f) to a restoration processing part 15, and this part 15 uses data (c) for restoration to restore and output input data (a).