  We study an inflation model with a nonlinear sigma field which has $SO(3)$
symmetry. The background solution of the nonlinear sigma field is proportional
to the space coordinates linearly while keeping the homogeneous and isotropic
background spacetime. We calculate the observable quantities including the
power spectra of the scalar and tensor modes, the spectral indices, the
tensor-to-scalar ratio, and the running of the spectral indices, and then
constrain our model with the Planck 2015 observational data.
