Japanease legal control of HC and NOx emissions from motor vehicles have a sufficienteffect on controling photochemical smog in HC-NOx reaction system. This thing was obtained through numerical simulations in effect equivalent to dynamic smog chamber tests using a simplified box model consisting of a reaction module and some environmental factors. Concerning the dilution capacity of R=100 %/h, if the emission flux of NOx is controled so as not to exceed 0.058 ton/km3h, the maximum concentrations of O3 and NO2, can be lowered to 0.10 and 0.023 ppm. More over, simultaneous control of both the HC and NOx so as not exceed 0.58 and 0.27 ton/km3h respectively, is predicated upon meeting both the Japanese O3 and NO3 air quality standards. It turns out that this control admits ample traffic volume 6100 cars/km2, and is a considerably more cost-effective strategy.