A shift control apparatus for an automatic transmission of a motor vehicle, including a sensor for detecting a throttle valve opening which regulates an intake air quantity supplied to the vehicle engine, an engine speed sensor for detecting the engine speed, a vehicle speed sensor for detecting the vehicle running speed, a calculating device for calculating the intake air quantity for a steady running of the vehicle, based on the detected throttle valve opening and engine speed, a processing device for processing the calculated intake air quantity to compensate the calculated intake air quantity for a first-order time lag and obtain an estimated intake air quantity, and an automatic shift control device for automatically selecting an optimum operating position of the automatic transmission, based on the estimated intake air quantity and the detected vehicle running speed, according to a predetermined shift control pattern.