A system for developing the preceding logic to be implemented in a built-in microcomputer that is used while being incorporated in an electronic control unit. The system includes: a motherboard having a first CPU, a first memory, and a first interface via which the motherboard communicates with the outside, interconnected over a first internal bus; a core board having a second CPU, a second memory, quasi microcomputer peripheral devices, which simulate by software the peripheral devices of a microcomputer, and a second interface via which the core board communicates with the outside, interconnected over a second internal bus; and a PCI bus that links the motherboard and the core board. The development system is substituted for the built-in microcomputer in order to implement the preceding logic.