Flow passage structure of fuel cell system

A flow passage structure employed in a fuel cell system to supply to a fuel cell stack a hydrogen gas supplied from a hydrogen gas supply source and a hydrogen off-gas supplied from a hydrogen pump, the flow passage structure comprising:

• a hydrogen gas pipe connected to the hydrogen gas supply source;
• a hydrogen off-gas pipe connected to the hydrogen pump;
• a supply pipe which connects the hydrogen off-gas pipe and the fuel cell stack to each other, into which a leading end part of the hydrogen gas pipe is introduced, and through which a mixed gas of the hydrogen gas from the hydrogen gas pipe and the hydrogen off-gas from the hydrogen off-gas pipe is supplied to the fuel cell stack;
• a lead-out pipe that is disposed inside the supply pipe along a central axis of the supply pipe to lead out the hydrogen gas from the hydrogen gas pipe into the supply pipe, the lead-out pipe including a first wall located between an inner wall and the central axis of the supply pipe, a second wall connected to the first wall and located between the first wall and the central axis, and a first region surrounded by the first wall and the second wall, the lead-out pipe being configured such that a second region between the inner wall and the first wall and a third region on a side of the central axis from the second wall communicate with each other;
• and
• a junction at which the first region and the leading end part of the hydrogen gas pipe communicate with and connect to each other, wherein
• the lead-out pipe is formed such that a length of at least one of the first wall and the second wall along the central axis is shorter at a position in a leading end part of the lead-out pipe that corresponds to a position of the junction than at a position that does not.