When an encrypted program and a decryption program are inputted to a first memory, a semiconductor integrated circuit device causes a bus port to disable access from the outside and enables access to the first memory and to a second memory, thereby transferring the encrypted program and the decryption program from the first memory to the second memory. When the transfer is completed, the semiconductor integrated circuit device disables access to the first memory and gives, to a CPU, an instruction to decrypt the encrypted program by using a secret key held in a secret key holder and the decryption program and execute the decrypted program. After the execution of the decrypted program is completed, the semiconductor integrated circuit device disables access to the second memory.