  We study the effects of sudden change in the sound velocity on primordial
curvature perturbation spectrum in inflationary cosmology, assuming that the
background evolution satisfies the slow-roll condition throughout. It is found
that the power spectrum acquires oscillating features which are determined by
the ratio of the sound speed before and after the transition and the
wavenumeber which crosses the sound horizon at the transition, and their
analytic expression is given. In some values of those parameters, the
oscillating primordial power spectrum can better fit the observed Cosmic
Microwave Background temperature anisotropy power spectrum than the simple
power-law power spectrum, although introduction of such a new degree of freedom
is not justified in the context of Akaike's Information Criterion.
