Dielectric behavior was examined for ternary blends of guest cis-polyisoprene (PI) chains, coguest PS chains, and host PS-PI-PS triblock copolymers. The guest PI chains had the molecular weight Mg-PI well below Mb-PI of the PI blocks and were mixed in the spherical PI domains formed in the copolymers. PI has the type-A dipoles parallel along the chain backbone and its end-to-end vector fluctuation activates dielectric relaxation. The dielectric test was conducted at temperatures well below the glass transition temperature of the PS blocks, so that the PI blocks had their ends fixed on the glassy PS matrix and exhibited no end-to-end vector fluctuation. Thus, the slow dielectric response of the systems exclusively detected the global motion of the guest PI chains confined in the spherical PI domain. The dielectric mode distribution of the guest PI chains thus observed was broader than that of the same chains in bulk systems, reflecting the osmotic and spatial constraints on the chain dynamics in the spherical domains. The guest PI chains of different Mg-PI exhibited nearly the same extent of mode broadening, despite their average coil size approached the domain size with increasing Mg-PI This result suggested that the osmotic constraint was the primary factor affecting (broadening and retarding) the guest chain dynamics. The osmotic constraint appeared to be stronger in the spherical domain than in the lamellar domain, the latter being examined in literature.