  We investigate primordial gravitational waves and curvature perturbations in
de Rham-Gabadadze-Tolley (dRGT) bimetric gravity. We evaluate the power-spectra
in the leading order in slow roll. Taking into account the decay of massive
graviton, we find that the action up to the second order reduces to the
Einstein theory with a non-minimally coupled scalar field, which is simplified
to a minimally coupled model by conformal transformation. We also find that the
tensor to scalar ratio for large field inflation with power law potential is
larger than the general relativity counterpart for any choice of parameters in
dRGT bimetric gravity. In addition, we confirm that the usual consistency
relation holds and we have a steeper spectrum for gravitational waves.
