An electronic shopping method is provided by both an apparatus 10 and an apparatus 30 to hold a key K(A, C) in common, and both the apparatus 30 and an apparatus 40 to hold a key K(B, C) in common. The apparatus 10 transmits a cipher text C1 which is obtained by enciphering a written order P of the products to be purchased using a key K, a cipher text C2 which is obtained by enciphering the key K using the key K(A, C), and authentication information C3 relating to a part P2 of the written order P to the apparatus 30 and the like. The apparatus 40 deciphers the key K on the basis of the cipher text C2, transmits a cipher text C4 which is obtained by enciphering the key K using the key K(B, C) to the apparatus 30. The apparatus 30 deciphers the key K on the basis of the cipher text C4, deciphers the written order P on the basis of the cipher text C1, authenticates the part P2 by referring the authentication information C3, and transmits a fifth cipher text C5 which is obtained by enciphering the authentication information C3 and the part P2 using the key K(B, C) to the apparatus 40. The apparatus 40 deciphers the part P2, and authenticates the part P2 by referring to the authentication information C3.