  The cross section for the reactions of the type $\mu^-\mu^+ \to e\bar\nu_e X$
can not be calculated by the standard methods due to the $t$--channel
singularity in the physical region. In this letter we show that accounting for
the finite sizes of the colliding beams results in the regularization of this
singularity. The finite cross section, which is obtained in this way, turns out
to be linear proportional to the transverse sizes of the colliding beams. As an
application of the above result, we calculate the cross section of the $W$
boson production at $\mu^+\mu^-$ colliders in the reaction $\mu^- \mu^+ \to
e\bar\nu _e W^+$.
