To provide a fluid mixer capable of uniformly mixing a second fluid B with a first fluid A over the whole flowing passage 4, while restraining an increase in a pressure loss to the first fluid A in the flowing passage 4, when mixing by jetting the second fluid B from a jetting port 5 to the first fluid A flowing in the flowing passage 4.

This fluid mixer has a spiral guide vane 1 formed by projecting to the inner diameter side from a wall surface 6 of the flowing passage 4 and having a spiral shaft in the communicating direction of the flowing passage 4. A communicating opening area 7 continuing in the communicating direction of the flowing passage 4 is formed in an inner diameter side part of this spiral guide vane 1. The jetting port 5 is arranged on the wall surface 6 of the flowing passage 4 upstream of a position of the spiral guide vane 1.