The electrostatic correlation lengths xi in salt-free polyelectrolyte solutions have been measured as functions of charge density and polymer concentration using a small-angle X-ray scattering (SAXS) technique. In this case a water-soluble poly(vinyl alcohol) (PVA) was employed as a parent polymer to avoid increasing hydrophobic interactions with decreasing the charge density; partially sulfuric acid esterificated PVA's were used as samples. The charge density defined by degree of esterification alpha was changed hom 0.008 to 0.499. It was found that the maximum position q(m) of a characteristic SAXS peak, which is attributable to the electrostatic interchain correlation, is proportional to the square root of polymer concentration C at any charge density as long as the solution is in the semidilute region. This supports that the isotropic model by de Gennes et al. is valid. The alpha dependence of q(m) agreed with the theoretical prediction from a blob chain model for weakly charged polyelectrolytes proposed by Pfeuty and Khokhlov. Thus, it was experimentally shown that, when the counterion condensation does not take place (alpha < 0.3), the relation q(m) similar to xi(-1) similar to alpha(1/3)C(1/2) is valid for semidilute solutions. This also indicates that the charge density dependence of the correlation length can be understood within a framework of the isotropic model.