We report a new insight into the chemical gelation process of poly(N-isopropylacrylamide) (PNiPAAm) hydrogel elucidated on the basis of the in situ observations with time-rested Fourier-transform infrared spectroscopy (FTIR), its principal component analysis (PCA), two-dimensional correlation spectroscopy (2D COS) at preparation temperatures above the lower critical solution temperature of PNiPAAm aqueons solution. First, we spectroscopically identified for the first time the specific time-spans for the two-stage reaction process: the first-stage giving rise to linear and branched random copolymers NiPAAm and cross-linker monomers and the second-stage giving rise to cross linking into macroscopic network structure. Second, we conducted 2D COS for both the first-stage and second-stge, reaction processes in order to resolve highly superposed spectra in the amide I and II regions into fundamental spectra comprising the superposed spectra. We elucidated the time-sequence order of the changes in the fundamental spectra occurring in situ during the gelation process. We proposed molecular interpretations of the sequence of the spectroscopic events, which would give deep insights molecular mechanisms of the gelation process as detailed in the text.