A control system for an automatic transmission, comprising: fluid transmission unit for receiving a rotation from an engine (10); a speed change unit (16) including an input clutch (CI) to be applied when a forward running range is selected, so that the rotation of the fluid transmission unit is inputted thereto when the input clutch is applied; a hydraulic control unit (40) for establishing a plurality of gear stages in the speed change unit. The hydraulic control unit (40) includes: a hydraulic servo (104) for applying/releasing the input clutch; a control signal oil pressure generating solenoid valve (105) for generating a control signal oil pressure; a control valve (106) for regulating a forward running range pressure; a change signal oil pressure generating solenoid valve (108) for generating a change signal oil pressure; and a change-over valve (107) adapted to be changed by the change signal oil pressure, for taking a first position, in which it feeds the forward running range pressure to the hydraulic servo, and a second position in which it feeds the control oil pressure to the hydraulic servo. The control unit includes: shift output generating means (115) for generating a shift output to achieve the individual gear stages; pressure reducing means (116) for outputting, when the stop state of the vehicle is detected by the stop state detection means (110), a signal to the change signal oil pressure generating solenoid valve (108) to bring the change-over valve (107) into the second position and reduce the control oil pressure thereby to bring the input clutch into a released state; pressure boosting means (117) for outputting, when the starting operation is detected by the starting operation detection means (111), the control signal to the solenoid of the control signal oil pressure generating solenoid valve (105) to boost the control oil pressure thereby to bring the input clutch into a predetermined applied state; fail decision means (118) for deciding a fail if the clutch input/output R.P.M. difference between the R.P.M. of the input side (101) and the R.P.M. of the output side (102) of the input clutch is over the set value which is set on the basis of the clutch input/output R.P.M. difference when the starting operation is detected by the starting operation detection means (111); and fail-safe means (119) for causing the shift output generating means to generate a shift output to a higher speed gear stage.