A controller of an automatic transmission can be set to be in a first control mode and a second control mode for controlling a transmission gear ratio of the automatic transmission. The controller in the second control mode automatically changes the transmission gear ratio. The controller obtains an index value and upper limit speed of rotation speed of a transmission output shaft. The controller in the first control mode allows the transmission gear ratio to be changed when an operation member is operated to increase the transmission gear ratio and the index value is equal to or lower than the upper limit speed. The controller has a setting section that sets the upper limit speed. The setting section sets the upper limit speed based on vehicle acceleration when the controller is in the first control mode and the operation member is operated to increase the transmission gear ratio.