There is described a sensor unit (3) for a rotary encoder which serves for detecting the rotary movements of a drive shaft (1) in both directions and which has a single-stage transmission with an input gear (15) which is arranged concentrically with respect to and is non-rotatably connected to the drive shaft (1). The sensor unit includes an absolute segment counter which serves for counting the angle segments through which the drive shaft passes within each 360° rotation and has the following components: a magnetic segment counter exciter arrangement non-rotatably mounted on the second gear (16) of the transmission, that engages into the input gear, a stationary segment counter sensor arrangement which includes a Wiegand or pulse wire (28) with wound-on induction coil (29) and which delivers electrical pulses whenever the magnetic segment counter exciter arrangement passes through predetermined angular positions, and an electronic circuit (32) which counts said electrical pulses, stores the respectively acquired count value and calculates therefrom the instantaneous angular position of the drive shaft having regard to the transmission ratio of the transmission.