To provide a data processing system that can enhance mutual authentication capability, when using a common key.

A storage unit 61 stores master key data, a storage unit 51 stores authentication key data, and mutual authentication units 53, 63 use a random number in 64 bits generated from a random number generating unit 60, to respectively select corresponding master key data and authentication key data. The mutual authentication unit 63 generates the authentication key data from the selected master key data and uses the data for a common key, in the case of making mutual authentication with the mutual authentication unit 53.