Because the swelling of polymer gels is a diffusion process of the polymer network, the swelling rate is determined by the diffusion coefficient of a polymer network. The diffusion coefficient (D) includes the polymer-solvent mixing contribution (D-mix) and the elastic contribution from the polymer network (D-el). In this study, we experimentally investigate D, D-mix, and D-el, using tetra gels whose network structural parameters are systematically controllable. We measured D and the shear modulus (G) using dynamic light scattering and dynamic viscoelastic measurement, respectively. According to the Tanaka-Fillmore theory, we evaluate D-mix and D-el using the linear relationship between D and G in the as-prepared state. Consequently, we demonstrate a relation of D = D-mix + D-el in the as-prepared, swelling, and equilibrium swollen states.