We developed the instrument to measure ambient NO_x concentrations in ppt level by laser induced fluorescence technique. Diode laser pumped solid state (DPSS) laser was used as the excitation source for NO₂ molecules its self and dye laser for NO₃ detection. We employed FAGE (fluorescence assay of gas expansion) technique to detect both NO₂ and NO₃ radicals. We achieved the ultra high sensitive NO₂ analyzer whoes LOD (limit of detection) was 1.8 ppt. We brought the instrument to the Rishiri island Hokkaido and tested the utility and found the high potential for the practical use. We also developed NO₃ sensor as an extension of NO₂ analyzer. We employed tunable dye laser as the excitation source for NO₃ molecule at 623 nm band. Successfully we applied NO₃ analyzer to detect N₂0₅ at the campus of Tokyo Metropolitan University in March 2004 because N₂0₅ was easily thermally decomposed into NO₃ and NO₂.