  We explore the perturbative QCD corrections to the inclusive semileptonic
decays of $B$ mesons. In inclusive $B$ decay processes the parton picture works
well and the scaling feature manifests because the mass of $b$ quark is larger
than the QCD scale. Due to this property, the decay rate may be expressed by a
single structure function describing the light-cone distribution of $b$ quark
apart from the kinematic factor. We derive a $q^2$ evolution equation for the
distribution function, which violates the scaling property through the
Altarelli-Parisi type approach. The evolution equation is numerically solved
and its phenomenological implications are also discussed.
