To gain insight into the nature of the pseudo-Jahn-Teller (JT) effect, an energy component analysis has been carried out for the bicyclic nonalternant hydrocarbons termed pentalenoid and heptalenoid systems by using the ab initio RHF method with 6-31G(d) basis set. Inspection of the energy component comprised in the total energy reveals that the stability of a less symmetrical nuclear configuration is largely responsible for the decrease in the internuclear repulsion energy and the interelectronic repulsion energy due to sigma electrons. These observations are consistent with an expansion of the carbon skeleton brought about by the pseudo-JT distortion. Another energy component also plays an essential role in the pseudo-JT stabilization : For the examples, the preference for the C-2h structure rather than the D-2h one for the pentalene arises from a decrease in the interelectronic repulsion energy due to pi electrons, while that for heptalene results from a lowering of the nuclear-electron attraction energy due to pi electrons. This sharp distinction between the pentalenoid and heptalenoid systems is accounted for in terms of an electrostatic interaction combined with a charge relaxation attributed to pi electrons.