Studies on the deformation mechanism of polymer networks with small-angle neutron scattering (SANS) were carried out for Tetra-PEG gels, that is, poly(ethylene glycol) (PEG) networks prepared from tetra-functional A- and B-type macromers consisting of PEG by cross-end-coupling. Tetra-PEG gels are hydrogels with narrow-dispersed polymer chains. The molecular weights of the prepolymers were 20k and 40k. SANS measurements were conducted for uniaxially stretched Tetra-PEG gels. Although the SANS patterns for deformed Tetra-PEG gel-20k showed small anisotropy, that is, prolate and oblate patterns with respect to the stretching direction at low and high-q regions, respectively, Tetra-PEG gel-40k exhibited no anisotropy at all in the low-q region, even in a gel stretched by five times. This indicates that Tetra-PEG gel-40k is a homogeneous network free from inhomogeneities, such as dangling chains and entanglements and can be regarded as an "ideal" polymer network. The deformation mechanism of Tetra-PEG gels was discussed in comparison with those for phantom Gaussian chain model and others, suggesting that the concentration fluctuations by thermal motions surpass the chain anisotropy introduced by deformation.