A fluid mixer which can mix fluid while making the distribution of concentration or distribution of temperature of the fluid in the direction of flow uniform without any unevenness and which is both compact and easy to install piping for is provided. This fluid mixer has a fluid inlet, a first flow path connected to the fluid inlet, a spiral flow path connected to the first flow path, a plurality of branch flow paths branched from the spiral flow path, a second flow path to which the plurality of branch flow paths are connected, and a fluid outlet connected to the second flow path, the plurality of branch flow paths branching from different positions of the spiral flow path and being connected with the second flow path at different positions of the second flow path.