In 1983, the Japanese Industrial Standard, Lowvoltage Three-phase Squirrel-cage Induction Motors for General Purpose (JIS C 4210) was revised to achieve an internationalization of that Standard. Main contents of the revision include the addition of B-class and F-class insulation to conventional E-class insulation, as well as regulations on torque characteristics, starting input characteristics, and noise level, in alignment with IEC and NEMA Standards. Since Japanese three-phase standard motors possess special characteristics of common use for both 50Hz and 60Hz, to realize frame assignment that enables use of the same horsepower as previously or even more, and to improve starting characteristics and efficiency, study and new technique on stray load losses, other than conventional design engineering, have been necessary. To reduce stray load losses, rotor slot insulation and nonmachining of rotor outer surfaces have been newly developed for mass-produced motors. This new manufacturing method is being applied to new types of motors. In applying these new methods, materials and their treatment for rotor slot insulation were developed that have proved to be appropriate for mass production and that reveal minimized dispersion. Also, studies on further improved accuracy of core dimensions have been conducted. Presented in this paper are the results of these studies.