The proper development of neural circuits is essential for normal brain function and is believed to be governed by the Hebbian rule a well-known hypothesis in neural circuit research, which suggests that neurons that fire together wire together. However, despite its long-standing history as a hypothesis, direct evidence supporting this notion and its influence on brain function has remained elusive. Our research investigated the Hebbian rule by experimentally inducing synchronous firing during the development of neural circuits in the mouse visual cortex. This brain region is known for its close relationship between neural circuit structure and brain function. To achieve this, we utilized a non-invasive transcranial optogenetic stimulation method, which allowed us to induce synchronous firing in ChR2-positive neurons during a critical period. We revealed a higher connection probability between synchronized neurons compared to ChR2-negative neurons or the control group without photostimulation, and this increase in connection probability was prevented by chronic treatment with MK801, an NMDAR antagonist. We used two-photon calcium imaging to examine the orientation selectivity of the stimulated neurons and found that synchronized neurons responded more similarly than the others. The increases in synaptic connectivity and similarity of orientation selectivity were not observed in randomly fired conditions. These outcomes suggest that developmental synaptic connection and its function depend not on the firing rate but on synchronicity.