An investigation of sound generated form a two-step reduction gearbox was carried out in order to obtain basic information for the reduction of sound and the abnormality diagnostics for gearboxes. Test gears having an accuracy of Class JIS 1, tooth profile error, and pitch error were prepared. Three kinds of tooth profile errors, particularly depression at the pitch point, and projection or recession at the addendum flank, were formed on the test gear teeth. The gearbox was driven with and without load, and the sound generated was picked up by a condenser microphone. Using the signal thus obtained, sound waveform observation, sound pressure level measurement, and real time frequency analysis etc. were carried out. From the experimental results and considerations, the fundamental characteristics of the sound of a gearbox having an erroneous gear were clarified, and some basic information on abnormality diagnostics for gearboxes was obtained.