Aiming to clarify spinodal decomposition of polymers in the induction period of crystallization, time-resolved small-angle x-ray scattering measurements have been made in situ for poly(ethylene naphthalate) while it was crystallized from the glass, or in the case of the so-called glass crystallization. It is confirmed for this polymer that in the very beginning of the induction period a scattering peak appears at around 0.03 A (-1) in scattering vector q, which corresponds to a characteristic wavelength of 200 A in density fluctuations, and grows with time. Time evolution of this scattering peak is described by the kinetics of the spinodal decomposition as previously reported for the glass crystallization of poly(ethylene terephthalate).