Abstract: Disclosed is an automatic transmission system in which an ECU calculates change of the running speed to automatically shift the gears under optimal conditions. The automatic transmission system used for heavy construction equipment for variably changing a rotative speed of an output shaft by selectively engaging any one of a plurality of change gears according to the selection of oil passages of a solenoid valve includes a speed sensor for sensing the rotative speed of the output shaft or a running speed of the heavy construction equipment, a converter connected to the speed sensor, for converting a value sensed by the speed sensor into an electrical signal and a controller for receiving the electrical signal output from the converter and comparing the electrical signal with a pre-stored value, to apply a driving signal to the solenoid valve if the electrical signal is within a predetermined range, so as to shift gears.