In an automatic transmission system having a selector and a generator responsive to the operation of the selector for generating electric information indicating the operation condition of the selector, the nature of the malfunction or trouble arising in the generator is discriminated on the basis of the change in the electric information. This discrimination may be made as to whether or not there is a malfunction condition wherein no information is obtained for any set position, whether or not there is a malfunction condition wherein the information indicating a specific position is always produced. When there is a malfunction therein, the position set by the selector is assumed by taking into account the nature of the malfunction, making it possible to control the gear-shift operation even if there is malfunction.