In a data transformation apparatus for transforming two arbitrary pieces of data of A input data and B input data, a first nonlinear transformation of the A input data is performed using a first key parameter, a transformed result is output, an XOR operation of the transformed result and the B input data is performed to output an XORed result as B intermediate data, and the B intermediate data is input to a next sub-transformation unit as B input data. On the other hand, the B input data is input to a next sub-transformation unit as A input data. A second nonlinear transformation of the B input data is performed using a second key parameter, the transformed result is output, an XOR operation of the transformed result and the B intermediate data is performed to output an XORed result as B intermediate data, and the B intermediate data is input to a next sub-transformation unit as B input data. The above configurations is connected in a cascade and the last A intermediate data and B intermediate data are output as transformation result.