To provide an information security device which executes a quick exponentiation arithmetic method like the small window method without enlarging the circuit scale of a coprocessor neither a memory.

A relatively large table required for the small window method which quickly performs exponentiation arithmetic is prepared on the outside of the coprocessor, and selection of a value from the table and transfer to the coprocessor are performed in parallel with multiple length arithmetic in the coprocessor. Two banks are prepared in the coprocessor as areas for multiple length values used for arithmetic, and one bank is used for arithmetic in the coprocessor while the other is used for data transfer. Banks are switched immediately when arithmetic in the coprocessor is terminated, and data transfer for next arithmetic is performed simultaneously with use of the already transferred value for arithmetic in the coprocessor.