The ground-state band structure of polydiacetylenes is theoretically studied with the extensional Su-Schriffer-Heeger model supplemented by electron-electron interactions. The results show the following. First, the interval of valence bands (conduction bands) increases because of the electron-electron interactions. Second, the effect of the on-site Coulomb energy (U) is different from that of the nearest neighbor Coulomb repulsion (V); the competition between U and V shows that U makes the bandwidth narrower and the gap broader, whereas V makes the bandwidth broader and the gap narrower. There is a critical value of UN. Third, the whole band width (E-omega) decreases when the UN ratio is less than 1.0 and increases when the UN ratio is greater than 1.0 at V = 2.0 eV. Thus, the ground-state band structure is sensitive to the UN ratio. The results also show that electron-electron interactions can play an important role in the band structure of polydiacetylenes.