Diffusion behavior of particles in hydrogels is important both for the fundamental understanding of mass transport and for practical applications and has been investigated for a long time. There are three major theories describing the diffusion behavior of small particles in a polymer network: obstruction, hydrodynamic, and free volume theories. Although many researchers have examined these three theories, their applicability is still unclear due to ambiguity stemming from the heterogeneity of conventional hydrogels. Recently, we have developed a near-ideal hydrogel called Tetra-PEG gel that provides a unique possibility to correlate gel structure with properties. In this study, we measured the diffusion coefficient of water molecules (D) in Tetra-PEG gels by pulsed field gradient spin-echo H-1 NMR. By comparing D and the correlation length of a polymer network (xi) measured by small-angle neutron scattering, we observed an identity formula similar to the hydrodynamic theory (D/D-0 = exp(-d/xi), where D-0 is the diffusion coefficient of particles in the absence of polymers and d is the diameter of a particle). This result suggests that the diffusion behavior of small particles in hydrogels is determined by the characteristic sizes of a particle (d) and a polymer network (xi).