An electronic circuit device has a high-density mount board (2), on which are disposed a microcomputer (3) and random access memory (7) which are connected to each other through an exclusive memory bus (12) for high-speed data transfer, a programmable device (8) which is a variable logic circuit represented by FPGA, and an electrically-rewritable nonvolatile memory (16) which can store the operation program of the microcomputer. The high-density mount board has external mounting pins on the bottom surface so that it can 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 can be simulated. With an operation program being written to the nonvolatile memory, a software-based function to be realized can be 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.