Effects of adding a small amount of a block copolymer to polymer blends composed of the same constituent polymers as the block copolymer on the kinetics of later-stage spinodal decomposition (SD) were investigated by a time-resolved light scattering at three different temperatures in a deep quench condition. Changes in the scaled structure factors F(x) = q(m)(t;T,phi)3I(x,t;T,phi) with x = q/q(m)(t;T,phi) as a function of the composition of the block copolymer phi were analyzed at various temperatures T, where q(m)(t;T,phi) is the magnitude of the scattering vector q at which the first-order peak of the scattering intensity profile I(q,t;T,phi) appears. It was found that the scaled structure factor obtained for each blend was almost identical, independent of the amount of the block copolymer. The time evolution of the global or the local structure is slowed with increasing block copolymer content. However, the dynamical self-similarity is held during the SD and the self-similar structure evolved is independent of phi, the amount of the block copolymer added to the blends, at least within the time range of our experiments and phi less-than-or-equal-to 0.06.