We achieved solution concentration change of a picoliter-sized water-in-oil (w/o) microdroplet reactor based on elec-trofusion and hydrodynamic breakup of microdroplets in a microfluidic channel (Fig. 1). A carrier droplet changes the solution concentration of a reactor droplet by electrofusion of them, and then they are divided without changing their reaction volume before and after the fusion-fission event. We found that this system realized high-speed concentration change of the reactor droplet (< 1 s even for complete exchange of solution). We believe that this system will be useful for analysis of dynamic chemical reactions (e.g., dynamic structural change of motor proteins, enzymatic reactions, etc.).