In order to obtain improved confinement plasmas with highradiation at high density, Ar gas was injected into ELMy H modeplasmas in JT-60U. A confinement improvement of HH98(y,2) ≈ 1 wasobtained with a high radiation loss power fraction (∼80%) at anelectron density of ∼0.65nGW. The HH factor was about 50% higherthan that in plasmas without Ar injection.