A fluid passage structure includes a first flow passage and a second flow passage crossing with each other via a crossing portion in a casing, the crossing portion including a pair of arc-shape recessed portions formed to be a pair of arc-shape lines arranged in line symmetrical positions with respect to an axis of the second flow passage in a view along an axis of the first flow passage, each arc-shape line being a part of a cross sectional contour of the first flow passage; and an inside projecting portion formed between the arc-shape recessed portions to project toward an inside of the first flow passage by connecting each end of the arc-shape lines, each end being closer to the axis of the second flow passage than the other end of a corresponding one of the arc-shape lines in the view along the axis of the first flow passage.