An electronic circuit device has a high-density mount board, on which are disposed a microcomputer, a random access memory, a programmable device which is a variable logic circuit represented by FPGA, and an electrically-rewritable nonvolatile memory which can store the operation program of the microcomputer. The high-density mount board has external mounting pins on the bottom surface so as to be mounted on a mother board in the same manner as a system on-chip multi-chip module. With an intended logic function being set on the programmable device, a hardware-based function to be realized by the electronic circuit device is simulated. With an operation program being written to the nonvolatile memory, a software-based function to be realized is simulated. Consequently, the device facilitates the debugging at early stages of system development, configures a prototype system, and contributes to the time reduction throughout the system development, prototype fabrication and large-scale production.