In my talk, we consider a modified gravity theory in order to explain the present-day acceleration of the expansion of the universe. Most of modified gravities theory fall into massive gravity theories with coupling. Naively, we should tune the mass of a graviton to the present Hubble parameter. However, in de Sitter spacetime with Hubble constant H, if the graviton mass "m" satisfies the ghost condition "m^2<2H^2", the helicity-0 mode of the graviton become a ghost mode which has negative energy. It is thought that the theory with ghost is inconsistent. In this seminar, I show that in a vacuum state the Lorentz symmetry is broken owing to the ghost. Then, the theory can be consistent.