In this contribution we submit a theory of vibrations of a tetra-atomic symmetric bent molecule ABBA in cis-conformation. Compared to the previous calculations, where some approximations have been made, now we gave a more rigorous version. For the molecule studied we have calculated frequencies of all five vibrations. Among them three vibrations, ax1, ay1, and sy1, are normal or nearly normal. The longitudinal vibrations are of two types: (→← →←) and (→→ ←←).