an automatic transmission control system includes a frictional engagement element formed of alternating radially inner and outer thin plates for selectively transmitting a torque . the control system further includes a hydraulic servo having a piston which selectively engages / disengages the frictional engagement element responsive to an oil pressure . an electric characteristic detector detects an electric characteristic which changes with change in the apply force of the frictional engagement element ; and an apply force control unit controls the apply force of the frictional engagement element . the electric characteristic detector includes a signal generator for generating an electric signal in accordance with the relative positions of two electrically conductive members , which move as the frictional engagement element is engaged / disengaged . the apply force control unit includes a hydraulic controller for controlling the oil pressure fed to the hydraulic servo , on the basis of the electric signal received from the signal generator .