This paper deals theoretically with the converting processes of energy from heat release into pressure mode generation. The converting rate is calculated about several representative types of heat release distribution. The main obtained results are as follows: (1) When heat is given to a gaseous medium, 1/κ parts of this energy are transformed into the entropy mode energy, and the remaining (κ-1)/κ parts are transformed into the pressure mode energy. The latter's ratio increases fot the case that the gas is flowing. (2) When the axial heat release distributing form is a unit rectangle pulse type, the converting rate is large in low frequency regions and diminishes variously in higher frequency regions. On the other hand, for sinusoidal and cosinusoidal types, the converting rate curves have different patterns. The one is similar to the unit rectangle pulse type, the other takes a curve such that the rate has low value in low frequency regions and high value in middle- or high- frequency regions. (3) From the obtained results, it was inferred that the oscillations of middle- and high-frequencies in combustion instability depend upon the length of heat release region and the magnitude of heat release oscillations and upon their distributing states.