Recently A lean-rich combustion is drawing attention as a way to decrease the level of NO_χ. But the lean-rich combustion burner has a problem that an aldehyde is generated in the ignition process. Numerical unsteady-state analysis was performed in order to make clear the aldehyde formation process in ignition of the methane-air lean-rich combustion flame. In the lean-rich flames, the lean and the rich slit burners were set side by side in the combustor. A two-dimensional numerical analysis was performed based on Skeletal Chemistry proposed by Smooke M.D. which includes 35 elementary reactions with 16 chemical species. We compared the 2 ignition cases. One case was that the combustion chamber was filled with air, the other case was that the combustion chamber was filled with mixture of fuel and air. So we found that the aldehyde was generated when the combustion chamber was filled with mixture of fuel and air.