We previously discovered that isotropic monomer solution shows birefringence due to its anisotropic structure after gelation in the presence of a small amount of rod-like polyelectrolyte. Here, we focus on what mechanism is responsible for the formation of anisotropic structure during gelation. Various optical measurements are performed to elucidate the structure change during gelation. The structure before and during gelation is in situ observed by scanning microscopic light scattering, confocal laser scanning microscope, polarizing optical microscope and depolarizing small angle light scattering. It is found that the existence of a large-size structure in monomer solution with the rod-like polyelectrolyte is essentially important to induce birefringence during gelation. The possible mechanism of the anisotropic structure formation during gelation seeded by a rod-like polyelectrolyte is proposed based on the results of these observations.