A laser beam emitted from a laser source is projected onto a target set in a vacuum chamber while being focused by a focusing optical system. This results in generating fast particles, such as protons and emitting the particles from the target. A light measuring device measures plasma emission from the target upon in-focus irradiation with the laser beam and an analyzing device analyzes a measurement signal therefrom to assess a generation state of fast particles. Then the focusing optical system and target are controlled through optical system moving mechanism and target moving mechanism on the basis of the result of the analysis and feedback control is performed on the generation state of fast particles in the target. This realizes a fast particle generating apparatus capable of monitoring the generation state of fast particles in real time and thereby efficiently generating the fast particles.