A calculation apparatus capable of executing any of a first calculating process operation including a first matrix calculation, and a second calculating process operation including a second matrix calculation, includes: a first calculation unit for executing the second matrix calculation; at least one calculation unit other than the first calculation unit, for executing a matrix calculation in parallel to the first calculation unit so as to execute the first matrix calculation; and a logic circuit for performing a logic calculation with respect to a calculation result of the first calculation unit and a calculation result of the other calculation unit. Then, when a calculation result of the first matrix calculation is requested, the calculation apparatus acquires the calculation result from the logic circuit. As a result, the calculation apparatus and an encrypt/decrypt processing apparatus can commonly perform portions of the plural calculating process operations which contain the matrix calculations, and can realize high speed operation by executing portions of the matrix calculations in parallel.