Abstract: A fluid mixer configured to stably supply a mixed fluid having a desired mixing ratio, and which can be reduced in size. This fluid mixer includes a Venturi tube having a constriction section at which the flow path area is reduced, the Venturi tube having formed therein a first inflow opening and a second inflow opening through which a second fluid flows into a low-pressure region produced due to an increase in fluid velocity when a first fluid passes through the constriction section. A valve body that is disposed within the Venturi tube, the valve body closing due to the pressure of the first fluid passing through the Venturi tube and changing the flow path area of the Venturi tube, blocking off the first inflow opening when closed, and opening the first inflow opening when opened; and a biasing part (elastic body) applying a biasing force in the valve closing direction of the valve body.