The paper presents a sensorless sliding mode observer for the flux magnitude of the induction motor (IM) drive. In order to implement a vector control scheme for the IM, the angle of the rotor flux and the flux magnitude are needed. To obtain them, it is typical to use a stationary frame flux observer - once the α, β flux components have been estimated, they are used to compute the flux angle and the flux magnitude. However, in Indirect Field Orientation methods, such an observer is not used; then, a separate flux magnitude estimator is needed. The paper presents a flux observer that is based on a modified IM model in the rotating reference frame. This is constructed on the skeleton of a sensored observer - in order to make it sensorless, the observer is fed with a speed estimate. The paper shows that, with a certain gain design, the resulting sensorless observer is insensitive to the potential input speed inaccuracy. The observer yields an accurate flux magnitude without measuring the speed.