A memory stores data, the contents of which must be kept secret. A memory stores security data and a latch circuit latches key data for releasing the security. A comparison circuit compares security data with key data, outputs a coincidence signal when they coincide with each other and outputs a non-coincidence signal when they do not coincide with each other. When a coincidence signal is output from the comparison circuit, an output signal of a change bit control circuit is set to the "0" level and address changing circuits change an address signal for reading out data from the memory according to the security data supplied from the memory.