Provided is a fluid mixing device which produces a series of solutions with a concentration gradient. The fluid mixing device includes: a plurality of first channels disposed parallel to each other on a layer, and into which an equal amount of diluent flows from its upstream; a plurality of second channels formed perpendicular to the first channels on an adjacent layer to the layer on which the first channels are formed, and into which an equal amount of sample solution flows from its upstream; and via holes formed at at least one intersection between each of the first channel and a plurality of second channels so that a predetermined amount of sample solution flows from the second channels into corresponding first channels, wherein a series of solutions with different concentrations is produced in the first channels depending on the amount of sample solution that flows into the first channels through the via holes. Thus, a series of solutions with different concentrations is output from the first channels.