A new conjugated oligomer was synthesized by reacting diphenyldiacetylene under high pressure of 0.03-0.13 GPa at 250-degrees-C for 5 h. The oligomer was obtained from reacted diphenyldiacetylene in liquid state and of a number-average molecular weight of 1200-1270 in polystyrene calibration. The fraction of the oligomer increased with increasing pressure. The structure of the oligomer was investigated from its visible absorption and infrared spectra, field desorption mass (FDMS) spectrum, thin-layer chromatography, and C-13 NMR spectrum. We found that the oligomer had a cyclic structure by the FDMS spectrum. The C-13 NMR spectrum for the oligomer corresponded to the polyacene-based structure. It was proposed that high-pressure oligomerization of diacetylene is a powerful method to produce a cyclic oligomer.