[en] A sheet-shaped thermal lithium beam probe has been developed for two-dimensional density measurements in the edge region of the torus plasma. For the production of an intense and uniform beam, the optimization of the nozzle shape and slit width of the beam injector has been performed. Beam density profiles at the observing region in the plasma were estimated with numerical calculations using a Monte Carlo technique for several kinds of nozzles and combinations with slits. Experiments on the test stand were also carried out, and pretty good agreement was found between numerical and experimental results. Based on these examination and optimization for the nozzle and slits, we have constructed a sheet-shaped beam injector for the large helical device. In spite of the long distance (about 3 m) from the injector, it is estimated that the thickness of the sheet-shaped beam is ≅90 mm (full width at half maximum). (author)