The photoionization cross-section of acetylene has a prominent double-hump structure, the peak positions of which are 13.3 and 15.3 eV. The assignment for the lower peak has long been controversial. In this Letter, we settle the problem by calculating the valence photoionization cross-section with the complex basis function method including the channel coupling in the random-phase approximation. At the neutral ground-state geometry, we assigned the lower peak to the 3 sigma(g) --> 3p sigma(u) Rydberg-type autoionizing state, which is perturbed by the 3 sigma(g) --> 3 sigma(u) intravalence excited state. We also studied the geometry dependence of the autoionizing state.