Resistance against simple power analysis is maintained while a smaller table is used. An IC card 100 decrypts encrypted information using elliptic curve calculation for calculating a point k*C by multiplying a point C on the elliptic curve E with a coefficient k that is a positive integer less that a prime p. The calculation of the point k*C is performed by adding a multiplication result obtained by multiplying a digit position (window) value w of the acquired coefficient k with the point C in a position corresponding to the digit position, and is performed with respect to all digit positions. When a non-negative integer exists that fulfills a condition that the acquired digit value w can be divided by 2t and cannot be divided by 2t+1, the multiplication includes adding a point obtained by multiplying a point Q with w/2t.