An apparatus for producing and automatically injecting H.sub.2.sup.15 O, having: a target box adapted to be irradiated with a deuteron beam, a gas supplying device for supplying a hydrogen gas and a nitrogen gas as raw-material gases to the target box a radioactive-isotope conveying pipe connecting the target box and a vacuum pump for maintaining the interior of the target box and the interior of the radioactive-isotope conveying pipe in a vacuum state. In addition, the apparatus has an injection-fluid producing device and an automatic injection device, wherein the target box, the injection-fluid producing device and the automatic injection device are fluidly communicated with one another.