A fluid mixer includes a main flow path comprised of a first flow path and a second flow path, spiral flow paths formed around the second flow path in shapes substantially concentric with the second flow path and offset in position from each other in a circumferential direction, the spiral flow paths having first ends communicated with the first flow path, branch flow paths branched from a plurality of locations of the second flow path in a flow direction, the branch flow paths being communicated with the spiral flow paths at a plurality of locations of the spiral flow paths in the flow direction, a fluid inlet at an open end of either of the first flow path and the second flow path, and a fluid outlet at an open end of the other of the first flow path and the second flow path.