The time-resolved photoluminescence spectrum of a polythiophene derivative has been analyzed from the viewpoint of the vibronic structure and its dynamics. We found that the photoluminescence (PL) originates from two vibronic coupled states which consist of the odd-parity 1B(u) component and the even-parity 2A(g) component. Since the fluorescences from the two states have different time constants and different vibronic structures, the PL shows a biexponential decay and a spectral change.