  In the framework of decay theory of Goldberger and Watson we treat
$\alpha$-decay of nuclei as a transition caused by a residual interaction
between the initial unperturbed bound state and the scattering states with
alpha-particle. The integrable wave function for the $\alpha$-decay is derived.
The alpha-particle is described by the wave packet, having small amplitude
inside the nucleus and exponentially growing in external region up to the
$\alpha$-wave front. The Moshinsky's distortions of the alpha-wave front are
analyzed. It is found that the energy of the decaying level does not satisfy
commonly accepted Bohr-Sommerfeld quantization rule for the quasibound levels.
Only far from this condition the decay rate turns out to be determined by the
Gamov's factor for the barrier penetrability. The derived general expression
for the decay rate is approximated by the familiar quasiclassical formula.
