We report 15N nuclear magnetic resonance (NMR) and the nitrogen-isotope effect studies on c-axis oriented sample of Li-doped HfNCl with superconducting transition temperature Tc~25.5 K. 15N NMR Knight shift decreases toward zero below Tc, giving evidence that the pairing symmetry is an even-parity spin singlet, and that the nitrogen site plays an important role in the occurrence of superconductivity. A nitrogen isotope shift is found to be quite small, ∇Tc~0.1 K (N = 0.07±0.02), which cannot reproduce such a high Tc within the traditional BCS framework.