We examined if air fuel ratio could be estimated by engine sound for a two-stroke glow engine. A time resolved FFT (Fast Fourier Transform) was adopted to analyze the engine sound. As a result, strong acoustic radiation were shown twice at TDC and BDC in the frequency range below 2.5 kHz in each cycle. For the case of high air fuel ratio, a peak of sound pressure in the range of 2.5 to 5 kHz exists in between TDC and BDC. This sound is supposed to be a noise due to anomalous combustion. Since the noise appears just after the combustion noise, the anomalous combustion noise is supposed to be a knocking noise. When the periodic noise in the range up to 2.5 kHz appears weakly twice in one cycle, the air fuel ratio is approaching low limit for a stable run of the engine at 12000 rpm. When the periodic noise in the range of 2.5 to 5 kHz appears once in one cycle between the periodic noise in the range up to 2.5 kHz, the air fuel ratio is approaching high limit.