The presence of both the upper critical solution temperature (UCST) and the lower critical solution temperature (LCST) has been found in poly(vinyl alcohol) (PVA)/water solution, which consists of 99% hydrolyzed PVA, 89% hydrolyzed PVA, and water: the solution had a LCST-type phase boundary at high temperature and an USCT-type boundary at low temperature. This immiscibility gap was determined by the cloud point measurement method maintaining the solutions for 2days at the desired temperatures. The coexistence of the UCST and the LCST was justified by the kinetic analysis of the phase separation. The increase of the apparent diffusion coefficient of spinodal decomposition (D_opp) obtained by the time-resolved light scattering method at high temperature region supports the existence of the LCST. On the other hand, the decrease of D_opp at low temperature region supports the existence of the UCST. The coexistence of both the UCST and the LCST was also supported by the supplemental experiments of temperature-jump and -drop from a two-phase region to a singlephase region.