The phase separation of a network in the unstable region has been investigated using a homogeneous transparent poly(vinyl alcohol) (PVA) gel by volume and UV transmittance measurements as well as by time-resolved light scattering measurements. It was found that the light scattering intensity I(Q) shows a single peak which increases in intensity exponentially with time without shifting the peak position in the early stage of phase separation, suggesting that a spinodal decomposition (SD) type phase separation occurs in this system. In contrast to the usual fluids, the domain growth is extremely slowed down in the late stage because the elastic force of the gel suppresses the surface tension force, which is the driving force of domain growth. This is the so-called pinning effect. As the SD approaches the end, the UV transmittance of the gel increases gradually and the scattering intensity decreases during the shrinking process of volume (syneresis), which corresponds to a relaxation process of the microscopic internal fluctuations produced in the SD process.