  We discuss graviton oscillations based on the ghost free bi-gravity theory.
We point out that this theory possesses a natural cosmological background
solution which is very close to the case of general relativity. Furthermore,
interesting parameter range of the graviton mass, which can be explored by the
observations of gravitational waves, is not at all excluded by the constraint
from the solar system tests. Therefore the graviton oscillation with possible
inverse chirp signal would be an interesting scientific target of KAGRA, adv
LIGO, adv Virgo and GEO.
