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This paper investigates the capability of long-baseline experiments, which are making use of neutrinos that are coming from Japan Proton Accelerator Research Complex, in establishing the unitarity of active-neutrino mixing by ruling out the nonunitary mixing scheme as a function of true values of CP-violating phase δCP. It is found that T2HK can establish the unitarity of active neutrino mixing at above 2σ C.L. irrespective of neutrino mass hierarchy and the true value of δCP, if the nonunitary (NU) parameter α21 is of the order of 10-2. Further, this paper also discusses the bound on the NU parameter in the 21 sector and the sensitivity limit of these experiments in determining the NU parameter. It is found that the bounds on (α21/2) are 0.028, 0.0026, 0.005 at 2σ 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. [ABSTRACT FROM AUTHOR]