An automatic transmission control system is provided with a disengagement malfunction determining section and a disengagement malfunction preventing section in an automatic transmission in which the frictional engaging elements are selectively engaged and disengaged to change a rotational speed of a main power source. The disengagement malfunction determining section determines a disengagement malfunction with a first frictional engaging element from among a plurality of frictional engaging elements of the automatic transmission. The disengagement malfunction preventing section prevents disengagement malfunction of a second frictional engaging element from among the frictional engaging elements of the automatic transmission by lowering the rotational speed of the main power source to a level which will prevent the disengagement malfunction in the second frictional engaging element, when the disengagement malfunction determining section determines that the disengagement malfunction of the first frictional engaging element exist while the automatic transmission is in non-running range.