Flow headers for small multiple pipes are commonly used in boilers and heat exchangers. The system contributes to raise the heat transfer efficiency in the components. The flow distribution mechanism of the header for water has been studied and the calculation procedure for the design has been recommended for a single-phase condition. It is also recommended to avoid the bubbles in the header to obtain a uniform water flow rate to each small pipe, but in some cases, the header has to be used to distribute a flow containing bubbles. Distribution behavior of water with or without a gas-phase was studied experimentally in a horizontal header with four vertical pipes. The prediction method developed for a single-phase fluid was extrapolated to the flow containing bubbles. The prediction agreed well with the experimental results at a small amount of bubbles.