To provide a device for quickly preparing the mixed liquid of a reagent in a proper amount and a concentration with a solvent, by inhibiting the development of foam in the mixed liquid.

This device for preparing the reagent is provided by equipping a container 6, a reagent-supply part 7 for supplying the reagent into the container 6, a solvent-supply part 8 for supplying the solvent B into the container 6 and a liquid surface sensor 9 for detecting the liquid surface of the mixed liquid A of the reagent and solvent, mixed in the container 6, and also equipping a solvent-supply port 14 for releasing the solvent B at the liquid surface of the mixed liquid A, formed in the container 6 so that the solvent B will not be directly put into the solvent-supply part 8.