A hydrogen producing system and a hydrogen producing method by which hydrogen can be efficiently produced using abundant seawater as the raw material. A hydrogen producing method has a system (10) for producing hydrogen from seawater, and the hydrogen producing system (10) has an activation device (12) and a pipe line device (14). The activation device (12) has a closed space (S) for introducing the seawater (W) and a vapor ejection means (18) for ejecting high-temperature, high-pressure vapor (T) into the closed space (S), and the activation device (12) activates under high temperature and high pressure the seawater (W) in the closed space (S). The pipe line device (14) is a device for receiving and leading the high-temperature, high-pressure seawater activated by the activation device (12) and includes one or more seawater leading tubes (403-408) having any one of triangular, pentagonal, hexagonal and octagonal cross sections.