PURPOSE: To protect the secrecy of the customer software stored in a ROM by inputting the ROM collation pattern data set outside a microcomputer to the microcomputer, comparing the collation pattern data with the ROM output data in the microcomputer, and outputting the built-in ROM data to the outside of the microcomputer when the coincidence is secured between the ROM collation pattern and the ROM output data.

CONSTITUTION: A ROM 11 is provided together with a program counter 12, an instruction decoder 13, the latches 14 and 16, a comparator 15, an input/ output switch circuit 17, a port 18, a coincidence flag 19, a one-shot pulse generating circuit 110, a counter 111, and AND circuits 112 - 114. At a ROM test of a microcomputer, the collation data equivalent to a built-in ROM of the microcomputer is prepared as an external test pattern. Then the ROM output data is outputted to the outside when the coincidence is secured between the ROM data and the collation data. Thus the outsiders are unable to arbitrarily read out the built-in ROM data. As a result, the security is protected for the customer software stored in a ROM.