AbstractA conceptual design of a DEMO reactor for the first integrated demonstration of generating fusion plant is made under the assumption that its design and construction would be started in 2020s and its operation in 2030s. A steady-state tokamak is minimized to have 5.8 m of major radius with 2.3 GW of fusion power with energy amplification Q exceeding 30. Modest extrapolation of improved plasma physics as well as technology development such as superconducting magnet, blanket and reduced activation material are assumed. It is suggested that demonstrating the fusion as a viable source of energy in plant scale will be possible based on the successful operation of ITER, while a number of technical issues must be solved in coherent programs paralleling ITER.