While a semiconductor memory operates in a first operation mode with high security, an encrypted command is inputted and then decoded to acquire the first address information. After the semiconductor memory comes into a second operation mode where the level of security is lower than that of the first operation mode, a command is inputted. Then, the second address information is acquired from the command. A control circuit in the semiconductor memory generates an address of 10 bits by using the first address information as a high-order 4 bits and the second address information as a low-order 6 bits and outputs the address to a memory array. With this operation, it becomes possible to read/write data from/to the memory array.