An electric power steering apparatus includes: a motor for providing assistant steering power; a following pulley connected with the motor through a belt to receive a driving force; a gear housing surrounding the following pulley; a motor housing rotatably assembled with the gear housing; and a tension adjusting mechanism for adjusting a rotation angle of the motor housing so as to adjust a tension of the belt. Therefore, the relative rotation angle of the motor housing respective to the gear housing can be adjusted so that the tension of the belt can be easily controlled and the generation of vibration and noise of the belt can be prevented.