A method to tune a basic input/output gain (BIOG) according to usage conditions for human-machine systems is proposed for improving work performance. Adapting a BIOG is effective in terms of improving operability and work efficiency, but frequent changes using a gain scheduling strategy degrade the operability by confusing the machine dynamics. The proposed tuning system adjusts a BIOG at long intervals on the basis of comprehensive features in operator and work content obtained from the histogram of control lever input. The target value is set to the normal distribution, meaning that all ranges of the control lever are evenly used in a spring-type lever, as leveling the histogram independent of an operator and work content provides the consistent operational feeling, which leads to comfortable operability. To ensure practicality and effectiveness, a BIOG curve is set to a polygonal line involving a break point and a saturation point that are tuned by equivalent transform of area differences between the obtained histogram and normal distribution curves. Two types of experimental task were performed using a hydraulic arm system. Results showed that the proposed BIOG tuning system improves time efficiency by reducing the area difference while increasing the subjective usability compared with a conventional fixed BIOG system.