PURPOSE: To protect the information stored in a ROM which is being tested, by inserting temperature fuses, which shut out data read out functions, into the data bus, in a hybrid integrated circuit having the ROM connected to a CPU and data bus terminals through which the data stored in the ROM are read out to the outside via the data bus.

CONSTITUTION: Temperature fuses 2 are inserted into a data bus 6 which connects address ROM data 5 of a hybrid integrated circuit 1 and a data bus 7. During a production process, the contents of the address 5 of ROM are read out at the data bus terminal 7 for the test and good products are selected. After that process, the products are put into a thermostatic chamber, the atmospheric temperature is raised, the temperature fuses 2 are melted and the circuit between the data bus 6 and the data bus terminals 7 is disconnected so that the contents of the data 5 of ROM can no longer be read out from the data bus terminals.