To provide a mixing device, a mixing tube, a drug solution injecting system, and a drug solution mixing method capable of evenly and efficiently mixing a plurality of kinds of drug solutions. The mixing device according to the present invention includes: a swirling flow generating chamber a first inflow opening for introducing a first drug solution into the swirling flow generating chamber in a direction parallel to a central axis of the swirling flow a second inflow opening for introducing a second drug solution into the swirling flow generating chamber so as to generate a swirling flow of the second drug solution having a specific gravity lower than that of the first drug solution an outflow opening for discharging a mixed drug solution and a narrowing chamber which is interposed between the swirling flow generating chamber and the outflow opening and has a space continuously narrowed toward the outflow opening.
