A plurality of user terminals or sensors transmit data encrypted by individual cryptographic key, a server receives the encrypted data items, and executes a data process according to a program defining a decryption process sequence. Bit slice expression data is generated by performing a bit slice process with respect to the plurality of encrypted data items which are decryption target, bit slice expression key based on the cryptographic key of each encrypted data item is generated, round key is generated based on a bit slice expression key, a decryption process including operation and movement processes of a block unit of the bit slice expression data, and an operation using the round key is executed, and a plurality of plain text data items corresponding to the plurality of encrypted data items are generated by a reverse conversion of the data with respect to the decryption process results.