  We discuss how much we can probe the effective number of neutrino species
N_nu with cosmic microwave background alone. Using the data of WMAP, ACBAR, CBI
and BOOMERANG experiments, we obtain a constraint on the effective number of
neutrino species as 0.96< N_nu <7.94 at 95% C.L. for a power-law LCDM flat
universe model. The limit is improved to be 1.39 < N_nu < 6.38 at 95% C.L. if
we assume that the baryon density, N_nu and the helium abundance are related by
the big bang nucleosynthesis theory. We also provide a forecast for the PLANCK
experiment using a Markov chain Monte Carlo approach. In addition to
constraining N_nu, we investigate how the big bang nucleosynthesis relation
affects the estimation for these parameters and other cosmological parameters.
