Power steering apparatus

The rotation conversion mechanism (50, 50', 150) in a power steering apparatus converts the relative rotation between input and output shafts (2, 102, 3, 103) connected by a torsion bar (6, 106) into relative rotation between a first rotating element (31, 111) and a second rotating element (32, 112). The ratio of the latter relative rotation angle to the former relative rotation angle is set as an invariable fixed value. An actuator (20, 120) is controlled by a control mechanism (30, 131) so that steering assist force which corresponds to the relative rotation angle between the two rotating elements (31, 111, 32, 112) is generated. When the relative rotation of the input shaft (2, 102) to the output shaft (3, 103) is converted into relative rotation of the first rotating element (31, 111) to the output shaft (3, 103), the direction of rotation is reversed by a reversing element (54, 54', 154).