The structure of the opaque poly(acrylamide) gels is studied by using a confocal laser scanning microscope. The polymer network of the gel consists of the fractal aggregate of the colloidal particles in the higher concentration region of the cross-linker. The diameter of the colloidal particle, which formed in the gel, increases from 180 to 420 nm with an increase of the concentration of cross-linker. On the other hand, the fractal dimensions of the aggregate remain constant, ranging from 1.5 to 1.7. The densities of the particle are calculated to be 0.7 and 1.2 x 103 kg/m3, which are >10 times larger than the average density of the polymer network of the gel. The results indicate that the monomer and the cross-linker are densely cross-linked into the particles.