The diagnostic system of JFT-2M has consisted of about 30 individual diagnostic instruments, which were used to study plasma production, control, equilibrium, stability, confinement, plasma heating by neutral beam injection and/or by radio-frequency (rf) (lower hybrid, ion cyclotron resonance frequency, electron cyclotron heating), and current drive by rf. In these instruments, the motional Stark effect polarimeter, charge-exchange recombination spectroscopy, heavy ion beam probe, time-of-flight neutral particle analyzer, etc., have helped in further understanding the improved mechanism of confinement such as H-mode and high-recycling-steady H-mode, and operational regimes of these modes. An infrared television camera system and a lost ion probe have played a very important role in investigating the heat load on the walls due to ripple lost particles and escaping ions from the core plasma region, respectively.