A photosensitive Oregonator that has three steady states (one is stable and the others unstable) is investigated in a one-dimensional reaction-diffusion system to study the decomposition of chemical waves in a closed excitable medium. A time-dependent concentration of reactant, a, plays the role of an internal bifurcation parameter. A part of the system is set to be oscillatory to produce a wave train. The chemical waves in the excitable medium become irregular with time, and before the system reaches a steady state a new oscillation mode characterized by a small amplitude and a high frequency emerges. This phenomenon implies that a basin of the unstable focus in the excitable Oregonator is perceptible in the pattern decomposition process-in the reaction-diffusion system.