PURPOSE: To provide the secret key protecting system and the cipher processing of this system where read/write of a memory part where secret key data is stored can be checked but secret key data cannot be observed from the external.

CONSTITUTION: Data in a memory part 1 where secret key information is stored can be read/written from the external in the test mode. A control part 2 which controls permission/inhibition of external output of read data from the memory part 1, a storage part 3 where all stored information are reset by input of a test mode signal TM and information related to addresses where data is written in the memory part 1 is stored thereafter, and a comparing part 4 which compares stored information of the storage part 3 and information related to the address for data read of the memory part 1 with each other are provided. When the test mode is set and the comparison in the comparing part 4 results in coincidence, the control part 2 permits data output. The memory part 1 and a cipher processing part which generates secret key information are unified and integrated into a cipher processing LSIC to improve the secrecy.