PURPOSE: An improver to reduce an amount of NOx evolved during combustion of gasoline only by adding a small amount of the improve to gasoline, comprising a lactic acid compound such as methyl lactate, etc.

CONSTITUTION: An amount of NOx is extremely reduced only by adding 0.0005wt% lactic acid compound such as methyl lactate, ethyl lactate, butyl lactage, etc. to gasoline. For example, when an amount of NOx is 386ppm in case of no addition of methyl lactate the amount is 162ppm in case of 0.0005wt% addition, 140ppm in case of 0.005wt% addition, 159ppm in case of 0.0005 addition of ethyl lactate, 148 ppm in case of 0.005wt% addition, 187ppm in case of 0.0005wt% addition of butyl lactate, and 166ppm of 0.005wt% addition.