A 500 keV, 10 MW neutral beam injector is to be constructed in JT‐60 Upgrade for the experiments of current drive and heating of heat density core plasmas. This is the first neutral beam injector in the world using negative ions as the primary ions. In the design, D−ion beams of 44 A, 500 keV are produced by two ion sources (22 A/each ion source) and neutralized in a long gas neutralizer. The total system efficiency is about 40%. The ion source is a cesium‐seeded multicusp volume source having a three stage electrostatic accelerator. To reduce the stripping loss of D−ions in the accelerator, the ion source should be operated at a low pressure of 0.3 Pa with a current density of 13 mA/cm2. The first test of the full‐size negative ion source is scheduled from middle of 1993.