Conformations of highly purified ring polystyrene, R-70, with the molar mass of 70 kg/mol, in a good solvent and in linear polymer homologue matrices were examined by small-angle neutron scattering (SANS) measurements. The radii of gyration R-g of R-70 were estimated by the Guinier's approximation from the SANS profiles obtained, and the polymer volume fraction Phi dependence of R-g(2) was discussed. In deuterated toluene as a good solvent, R-70 exhibits the R-g(2) similar to Phi(-0.29 +/- 0.01) dependence at high Phi above the overlap volume fraction, Phi(0)* (i.e., 1