  We show that UV photons produced by decaying particles can partially reionize
the universe and explain the large optical depth observed by Wilkinson
Microwave Anisotropy Probe. Together with UV fluxes from early formed stars and
quasars, it is possible that the universe is fully ionized at z \lesssim 6 and
partially ionized at z \gtrsim 6 as observed by Sloan Digital Sky Survey for
large parameter space of the decaying particle. This scenario will be
discriminated by future observations, especially by the EE polarization power
spectrum of cosmic microwave background radiation.
