  Using the band structure calculation and mean-field analysis of the derived
three-chain Hubbard model with phonon degrees of freedom, we discuss the origin
of the orthorhombic-to-monoclinic phase transition of the layered chalcogenide
Ta$_2$NiSe$_5$. We show that the Bose-Einstein condensation of excitonic
electron-hole pairs cooperatively induces the instability of the phonon mode at
momentum $q\rightarrow 0$ in the quasi-one-dimensional Ta-NiSe-Ta chain,
resulting in the structural phase transition of the system. The calculated
single-particle spectra reproduce the deformation of the band structure
observed in the angle-resolved photoemission spectroscopy experiment.
