  This paper investigates the capability of long-baseline experiments, which
are making use of neutrinos that are coming from Japan Proton Accelerator
Research Complex (J-PARC), in establishing the unitarity of active-neutrino
mixing by ruling out the non-unitary mixing scheme as a function of true values
of CP-violating phase $\delta_{\mathrm{CP}}$. It is found that T2HK can
establish unitarity of active neutrino mixing at above 2$\sigma$ C.L.
irrespective of neutrino mass hierarchy and true value of $\delta_{CP}$, if
non-unitary (NU) parameter $\alpha_{21}$ is of the order of $10^{-2}$. Further,
this paper is also discuss the bound on NU parameter in 21 sector and
sensitivity limit of these experiments in determining NU parameter. It is found
that the bounds on $\left(\alpha_{21}/2\right)$ are 0.028, 0.0026, 0.005 at
2$\sigma$ C.L. respectively for T2K, T2HK, and T2HKK. Moreover, it is also
found that the sensitivity limit of T2HK on NU parameter is far better than
that of both T2HKK and T2K.
