Automatic transmission control system and control method have the functions or steps of: controlling shifting of an automatic transmission installed on a vehicle, in one of an automatic shift mode in which shifting of the automatic transmission is carried out based on a shift diagram, and a manual shift mode in which shifting of the automatic transmission is manually carried out; detecting at least one shift command of an upshift command for shifting up the automatic transmission and a downshift command for shifting down the automatic transmission, the shift command being manually entered; measuring a shift command time for which the shift command is detected; and switching from the automatic shift mode to a shift inhibition mode in which shifting of the automatic transmission is inhibited, without switching to the manual shift mode, based on the measured shift command.