By combining the transient thermohydrodynamiccalculation feature from a general purpose program writtenfor single-phase fluids and an optional feature of gas-liquidphase change simulation (condensation simulation), acondensation simulator that is easy-to-use as a development tool has been developed, while achieving qualitativeprecision. The simulator makes it possible to understandand visualize the condensation process that takes place in aminute area, which is impossible to measure in actualproduct evaluations. It will also be used in the future tosolve problems in optimizing moisture-proof structures anddeveloping new moisture-proof treatments for electronicproducts.