In a preburner of liquid rocket engines, some liquid-oxygen (LOX) posts, which introduced oxygen and hydrogen into combustion chamber, experienced severe flow-induced vibration in unsteady hydrogen flow. The mechanism of the vibration has not been fully understood because of the complexity of the flow field. In the present study, a new numerical method was developed to analyze the flow field of hydrogen, whose characteristic properties lie in its non-idealness and compressibility. The unsteady hydrogen flow inside the preburner unit was analyzed to investigate the details of hydrogen flow field as well as the mechanism of LOX post vibration. It was clarified that hydrogen flow inside the manifold was strongly affected by the upstream condition.