At a position between a fuel offgas inlet portion and a fuel gas inlet portion in a main body portion in the facing direction where a first end faces a second end, a fluid confluence joint is provided with at least either one of (i) at least one step formed over a whole circumference of an inner wall of the main body portion by reducing the passage sectional area on a fuel gas passage portion side to be smaller than the passage sectional area on a confluence passage portion side, and (ii) at least one partition wall formed over the whole circumference so as to project inwardly from the inner wall of the main body portion.