The most common approach to classify the flow, which can be found in various reactors, is to divide into 'single phase flow' and 'multi phase flow'. Water flow without boiling, or air flow is typical example of single phase flow. In thermal design of nuclear reactor, it is often the case that 'gas-liquid two phase flow' or single phase flow is treated. Therefore, the focus of our study is on development of the simulation code, which can treat these two kinds of flow.