A position sensor for an automatic transmission has multiple switches, each of which generates “ON” or “OFF” signal depending on a selected position of a shift lever. Signal patterns (“ON” and “OFF” signals) of the normal operation are memorized as a map of signal patterns in a first memory portion. A second memory portion memorizes the respective signal patterns at the respective selected positions of the shift lever. The signal pattern and the change of the signal patterns in the second memory portion are compared with those memorized as the map of the signal pattern in the first memory portion, and determines that a malfunction has occurred in the position sensor, if at least one of the signal patterns and the changes of the signal patterns does not coincide with any of those memorized the map of the signal pattern. The malfunction of the position sensor can be surely detected at an earlier stage of operation of the shift lever for the automatic transmission.