A plasma diagnosis system using multiple-reciprocating-path Thomson scattering, comprising: a light source which supplies pulsed laser beams having predetermined polarization and wavelength; an optical system which sequentially supplies a pulsed laser beam in a vertical polarization state and a pulsed laser beam in a horizontal polarization state by using the pulsed laser beams supplied from the light source; a collection optic system which measures a first collection signal scattered from the plasma when the pulsed laser beam in the vertical polarization state supplied from the optical system is focused and measures a second collection signal scattered from the plasma when the pulsed laser beam in the horizontal polarization state supplied from the optical system is focused; and a controller which measures a Thomson scattering signal for the plasma by using the first and second collection signals measured by the collection optic system, wherein the optical system includes: a polarizing beam splitter, PBS, which is disposed on an optical path of the pulsed laser beam supplied from the light source; a first reflecting mirror which supplies the pulsed laser beams reflected from the PBS back to the PBS; a Faraday rotator which is disposed on the optical path of the pulsed laser beam passing through the PBS and rotates the polarization state by 45 degrees and outputs the pulsed laser beam; a focusing lens which focuses the pulsed laser beam supplied from the Faraday rotator on the plasma; and a second reflecting mirror which reflects and supplies the focused pulsed laser beam back to the focusing lens, and wherein the pulsed laser beam in the vertical polarization state and the pulsed laser beam in the horizontal polarization state are sequentially supplied to the plasma; wherein the first collection signal is a signal in which a Thomson scattering signal and a background scattering noise signal are mixed, and the second collection signal is a background scattering noise signal; and wherein the controller accurately measures only the pure Thomson scattering signal by removing the second collection signal from the first collection signal. a light source which supplies pulsed laser beams having predetermined polarization and wavelength; an optical system which sequentially supplies a pulsed laser beam in a vertical polarization state and a pulsed laser beam in a horizontal polarization state by using the pulsed laser beams supplied from the light source; a collection optic system which measures a first collection signal scattered from the plasma when the pulsed laser beam in the vertical polarization state supplied from the optical system is focused and measures a second collection signal scattered from the plasma when the pulsed laser beam in the horizontal polarization state supplied from the optical system is focused; and a controller which measures a Thomson scattering signal for the plasma by using the first and second collection signals measured by the collection optic system, wherein the optical system includes: a polarizing beam splitter, PBS, which is disposed on an optical path of the pulsed laser beam supplied from the light source; a first reflecting mirror which supplies the pulsed laser beams reflected from the PBS back to the PBS; a Faraday rotator which is disposed on the optical path of the pulsed laser beam passing through the PBS and rotates the polarization state by 45 degrees and outputs the pulsed laser beam; a focusing lens which focuses the pulsed laser beam supplied from the Faraday rotator on the plasma; and a second reflecting mirror which reflects and supplies the focused pulsed laser beam back to the focusing lens, and a polarizing beam splitter, PBS, which is disposed on an optical path of the pulsed laser beam supplied from the light source; a first reflecting mirror which supplies the pulsed laser beams reflected from the PBS back to the PBS; a Faraday rotator which is disposed on the optical path of the pulsed laser beam passing through the PBS and rotates the polarization state by 45 degrees and outputs the pulsed laser beam; a focusing lens which focuses the pulsed laser beam supplied from the Faraday rotator on the plasma; and a second reflecting mirror which reflects and supplies the focused pulsed laser beam back to the focusing lens, and wherein the pulsed laser beam in the vertical polarization state and the pulsed laser beam in the horizontal polarization state are sequentially supplied to the plasma; wherein the first collection signal is a signal in which a Thomson scattering signal and a background scattering noise signal are mixed, and the second collection signal is a background scattering noise signal; and wherein the controller accurately measures only the pure Thomson scattering signal by removing the second collection signal from the first collection signal.