System for and method of controlling automatic transmission

A control system and method of an automatic transmission for determining and setting a gear stage on the basis of at least two of the parameters including a vehicle speed and a throttle opening. The control system comprises: a first detector (1) for detecting a first parameter; a second detector (2) for detecting a second parameter; a gear stage memory (3) for determining and storing speed stage areas for holding the gear stages individually in terms of the first and second parameter and any two speed stage areas in a partially overlaid manner; a shift decider (4) for determining a shift to be executed, on the basis of the first parameter outputted from the first detector (1), the second parameter outputted from the second detector (2), and the stored content of the gear stage memory (3); and a gear stage determiner (5) for determining a gear stage to be set, if there are a plurality of gear stages determined by the first parameter and second parameter detected, at any of the gear stages.