We propose a new scenario for supermassive star (SMS; ≳10 5M⊙) formation in shocked regions of colliding cold accretion flows near the centers of first galaxies. When the post-shock density is high enough for collisionally exciting H2 rovibrational levels (≳104cm-3), enhanced H2 collisional dissociation suppresses the gas to cool below 8000 K. In this case, the layer fragments into massive clouds (≳105M⊙), which collapse isothermally (∼ 8000 K) by the Lyα cooling without subsequent fragmentation. As an outcome, SMSs are expected to form and evolve eventually to seeds of supermassive black holes. By calculating thermal evolution of the shocked gas, we delimit the range of post-shock conditions for the SMS formation. We also find that metal enrichment does not affect the SMS forming condition for ≲10-3Z⊙ if metals are in the gas phase.