To provide a microcomputer chip capable of preventing the unauthorized rewriting of control software when the microcomputer chip in a form with a sub-chip unconnected thereto is installed in an ECU in a completed state, and capable of efficiently performing the rewriting of the control software when the microcomputer chip in a form with a sub-chip connected thereto is installed in an ECU in a development stage.

A calibration chip 2 is connected through a terminal 22 to a main chip 1 of a microcomputer to be installed in an ECU in the ECU development stage. Then, the main chip 1 is provided with a terminal 23 set at a predetermined voltage when the chip 2 is connected. In the main chip 1, furthermore, when a rewrite request is received from a program rewriting tool outside the ECU, a CPU 12 determines whether or not the chip 2 is connected from a voltage level of the terminal 23 determined by an I/O port 16, and when the chip is determined to be connected, the CPU performs the rewriting of control software without performing authentication processing for determining whether or not authentication data specific to the type of ECU are transmitted from the tool.