Towards understanding of the inclusive vs exclusive puzzle in the $|V_{cb}|$ determinations

Lattice calculation may play an important role to understand the cause of the long-standing puzzle among the determination of $|V_{cb}|$. The key element is the computation of the inclusive decay rate, for which the formalism is under development. The method has its own problem: the approximation of the phase-space factor and finite volume effect. I discuss the current status of such study and future prospects.