We have experimentally studied the effect of neutrals on the H-mode transition in JT-60U. The neutral density near the separatrix was evaluated with the DEGAS code. The effective edge ion collisionality right before the H-mode transition tends to decrease with the increase of . L- and H-mode phases are clearly separated in the - plane. This result suggests that the edge neutrals have a substantial influence on the condition of the H-mode transition.