  We study the linear perturbation of the recently proposed model of inflation
where a uniform gauge-kinetic coupling of the inflaton to multiple vector
fields breaks the cosmic no-hair conjecture while maintaining the isotropy. We
derive the general quadratic action for the perturbation and calculate the
power spectra of scalar and tensor modes at the end of inflation by in-in
formalism. It is shown that the model predicts slightly red spectra and the
tensor-to-scalar ratio tends to be suppressed. The comparison with the data
from WMAP 7-year does not impose strong constraints on the parameters and both
weak- and strong- gauge-field regimes are consistent with the current
observations.
