This study was conducted to detect faults in a gearbox comprised of gears, bearings and shafts. The envelope analysis was used as a method of detection, which could detect breakage and pitting of gears and bearings, and misalignment of shafts effectively. Vibration measured at the increaser was analyzed to characterize the faults. When the increaser has a defect, peaks with a constant period appear in the time history of vibration and its harmonic components also in the envelope spectrum. The envelope analysis showed that a crack in the stepped output shaft caused the increaser to generate the abnormal peaks.