A conceptual design study of a steady-state tokamak, JT-60 Super Upgrade, is being carried out. The capability of the present JT-60 facility will be fully utilized for this upgrade. The mission of JT-60SU is to establish integrated basis of physics and technology for steady-state tokamak reactors. In JT-60SU, steady-state physics will be evaluated in the intermediate parameter region between the present tokamaks and steady-state tokamak reactors. Technology development for long pulse operation and research for engineering safety will also be pursued.Recently, a discharge with Pp -3, PN -3 and H-factor -2 was sustained for about 1 s in JT-60U under the full current drive condition (bootstrap current -50 %, beam driven current -50 % at Ip = 0.5 MA, :e= 1.1 x lOI9 m-3) [l, 21. Here, &, is the poloidal beta value, PN is the normalized beta value and H-factor is the confinement enhancement factor defined by H = ZE / T E~~~~~. In addition, a current drive and current profile control by LH, in a bootstrap dominant discharge, were demonstrated (lp = 0.7 MA, ne= 1 x lOI9 m3). However, the plasma current, density and pulse length are too small compared with those of steady-state tokamak reactors like SSTR [3], so that it is essential to establish steady-state plasmas and to study its Characteristics in a longer pulse, higher current and higher density regimes. To pursue these goals and to perform the advanced research and development for a prototype reactor according to the 3rd Phase Basic Program of Fusion Research and Development laid down by the Atomic Energy Commission of Japan, JT-60 Super Upgrade (JT-60SU) is designed utilizing full capability of existing JT-60 facility [4,5]. The mission of JTdOSU is to establish integrated basis of physics and technology for steady-state tokamak reactors. These studies will also contribute to the steady-state burn in ITER as an ultimate goal.The physics goal is a simultaneous achievement of stable, steady-state and fully current-driven plasmas with high confinement, high bootstrap current fraction, dense, cold and radiative divertor function in reactor-relevant conditions. To study the steady-state physics, the pulse length of steady-state operation case is designed to be longer than 2000 s, which is sufficiently longer than the characteristic time of particle saturation of wall (30-60 s), diffusion of plasma current (70-130 s) and first wall temperature(500-1000 s). The steadystate physics issues are as follows:a) The required plasma' current and current drive product (Ipc%p"e) in SSTR is 12 MA and -300 x 1019 MA/m2, respectively. In bootstrap current dominant plasmas, the current profile control is important to obtain an MHDstable current profile because the bootstrap current is hollow and is determined by pressure profile which is difficult to control. The current profile control is also important to obtain high Confinement because the H-factor increases with the peakmg of current profile. JT-60SU will aim at fully current-driven plasma of U to 6 MA and goals are intermediate va...