It has been considered that the fundamental frequency of the exhaust noise of an internal combustion engine is the inverse of the number of firings per second. However, in most of the data measured with automobiles, there are many frequency components of 1/2, 1/4, or 1/6 of the fundamental frequency and their higher harmonics. An investigation on the generation of such frequency components is reported. It was found that the differences in the lengths of the exhaust pipes generate such frequency components. This paper describes, first, the reduction of the theoretical equation in which the phase modulation in the exhaust noise due to the difference in the length of the exhaust pipe of each cylinder generates such frequency components. The spectra which have the same frequency components as the spectra of the measured exhaust noise are obtained by numerical computation of the equation. The same results are also obtained by computer simulations. The difference in sound quality is felt by hearing the synthesized sound. It is expected that this study is usable in realizing an engine of agreeable exhaust noise quality.