Before content transmission, the content transmitting device and the content receiving device mutually authenticate each other to verify that the other device respects copyright and rightfully handles content, and then content is encrypted by shared key data and transmitted. It is arranged that in an authentication process, a time from transmission of an authentication request or a time from transmission of an authentication response until arrival of receipt acknowledgement data is measured and only when a measured time is less than a fixed upper-limit value, content is transmitted.