The structure of Tetra-PEG gel, a new class of biocompatible, easy-made, and high-strength hydrogel consisting of a four-arm polyethylene glycol (PEG) network, has been investigated by means of small-angle neutron scattering (SANS). Since the Tetra-PEG gel is prepared by cross-end-coupling two kinds of four-arm PEG macromers having different functional groups at the ends, i.e., amine group and succinimidyl ester group respectively, the coupling reaction occurs exclusively between PEG chains carrying different functional groups. SANS results showed that the four-arm PEG macromer aqueous solutions and Tetra-PEG gels were successfully described by the theoretical scattering function for multiarm Gaussian chains and the Ornstein-Zernike function, respectively. Surprisingly, no noticeable excess scattering that originated from cross-linking was observed in Tetra-PEG gels, suggesting that its network structure is extremely uniform. Investigations on nonstoichiometric Tetra-PEG gels showed weakening of the mechanical properties as well as an increase of dangling chains (defects) in the network. It is concluded that Tetra-PEG gels have an extremely uniform network structure, probably mimicking a diamond-like structure, and this is one of the reasons for the advanced mechanical properties of Tetra-PEG gels.