We study the dynamics of a polymer chain with gradually changing the solvent quality from good to poor by, dissipative particle dynamics simulation. We find several spectral modes related to internal motions of intrachain interaction. Approaching the coil-to-globule transition, point, all fast modes of spectrum omega > I (ns)(-1) disappear. There is only a slow mode at omega approximate to 0.66 (ns)(-1). Moreover, the spectral density at this slow mode reaches a maximum value at the transition point. We suggest that, at the transition point, the chain conformation relaxes to the most probable distribution only by the, slow mode. There is a critical slowing down of internal motion with passing through the transition point.