An electric power steering apparatus includes a reduction gear mechanism (17) for reducing the rotation speed of an output shaft (16) of an electric motor (15). The reduction gear mechanism (17) comprises an inscribed gear (39) rotatable interlockingly with the output shaft (16), and a circumscribed gear (40) in which the inscribed gear (39) is inscribed. An endless belt connects a driving pulley (41) integrally rotatable with the circumscribed gear (40) and a driven pulley (42) disposed so as to surround a steerable shaft. By heightening the reduction gear ratio by the speed reduction using the inscribed gear (39) and the circumscribed gear (40), the reduction gear mechanism (17) as a whole can achieve a high reduction gear ratio without making the diameter of the driving pulley (41) very small.