We study disparity between H \alpha and H \beta in early spectra of the type IIP supernova SN 2008in .
The point is that these lines can not be described simultaneously in a spherically-symmetric model with the smooth density distribution .
It is shown that an assumption of a clumpy structure of external layers of the envelope resolves the problem .
We obtain estimates of the velocity at the inner border of the inhomogeneous zone ( \approx 6100 km s ^ { -1 } ) , the filing factor of inhomogeneities ( \leq 0.5 ) , and the mass of the inhomogeneous layers ( \sim 0.03 ~ { } M _ { \odot } ) .
The amplitude of flux fluctuations in the early spectrum of H \alpha ( \Delta F / F \sim 10 ^ { -2 } ) imposes a constraint on the size of inhomogeneities ( \leq 200 km s ^ { -1 } ) .
A detection of fluctuations in the early H \alpha of type IIP supernovae might become an observational test of the inhomogeneous structure of their envelopes .
We propose also the indirect test of the clumpy structure of external layers : the study of properties of the initial radiation outburst due to the shock breakout .
The inhomogeneous structure of external layers of type IIP supernovae could be an outcome of density perturbations and density inversion in outer convective layers of presupernova red supergiant .