A drug solution injector for determining an accurate injection volume of a drug solution by weighing the drug solution reservoir with a weighing scale. A drug solution injector is provided with a weighing scale that is compact, does not disturb handling and enables an accurate determination of the injection volume of the drug solution. A drug solution injector includes a drug solution reservoir composed of a chamber for reserving a drug solution therein and pressurization mechanism for pushing out of the reserved drug solution. An injection line is connected to the reservoir to inject the drug solution to the body. The drug solution reservoir is fixedly provided with a weighing scale for measuring a weight of the drug solution stored in the drug solution reservoir.