A control device for an automatic transmission including a continuously variable transmission mechanism, an input element disposed between an internal combustion engine being a driving source of a vehicle and the continuously variable transmission mechanism, and arranged to rotate as a unit with the internal combustion engine, an output element arranged to rotate as a unit with an input shaft of the continuously variable transmission mechanism, and a torque converter having a lockup clutch disposed between the input element and the output element, the control device comprising: a control means (7,8) configured to control an engagement state of the lockup clutch and a transmission gear ratio of the continuously variable transmission mechanism in accordance with a traveling state of the vehicle; a rotation sensing means (80,84,8h) configured to sense a rotation speed difference (ΔN) between the input and output elements (11,21) of the lockup clutch; and a torque sensing means configured to sense an output torque of the internal combustion engine; the control means (7,8) including; a transmission gear ratio control section configured to perform a transmission gear ratio control to control the transmission gear ratio so that a rotation speed of the output element rapidly becomes closer to a rotation speed of the internal combustion engine when the sensed rotation speed difference (ΔN) sensed by the rotation sensing means (80,84,8h) becomes equal to or smaller than a predetermined value in a case where the lockup clutch is switched through a slip engagement state to a full engagement state, and a first engagement control section configured to perform a first engagement control to bring the engagement state of the lockup clutch to the full engagement state after the rotation of the internal combustion engine is increased with the engagement state of the lockup clutch in the slip engagement state while a torque transmission capacity of the lockup clutch is increased when the lockup clutch is switched through the slip engagement state to the full engagement state by an ON operation of an accelerator of the vehicle, characterized in that the control means (7,8) is configured such that the transmission gear ratio control is prohibited during the first engagement control by the first engagement control section. a control means (7,8) configured to control an engagement state of the lockup clutch and a transmission gear ratio of the continuously variable transmission mechanism in accordance with a traveling state of the vehicle; a rotation sensing means (80,84,8h) configured to sense a rotation speed difference (ΔN) between the input and output elements (11,21) of the lockup clutch; and a torque sensing means configured to sense an output torque of the internal combustion engine; the control means (7,8) including; a transmission gear ratio control section configured to perform a transmission gear ratio control to control the transmission gear ratio so that a rotation speed of the output element rapidly becomes closer to a rotation speed of the internal combustion engine when the sensed rotation speed difference (ΔN) sensed by the rotation sensing means (80,84,8h) becomes equal to or smaller than a predetermined value in a case where the lockup clutch is switched through a slip engagement state to a full engagement state, and a first engagement control section configured to perform a first engagement control to bring the engagement state of the lockup clutch to the full engagement state after the rotation of the internal combustion engine is increased with the engagement state of the lockup clutch in the slip engagement state while a torque transmission capacity of the lockup clutch is increased when the lockup clutch is switched through the slip engagement state to the full engagement state by an ON operation of an accelerator of the vehicle, a transmission gear ratio control section configured to perform a transmission gear ratio control to control the transmission gear ratio so that a rotation speed of the output element rapidly becomes closer to a rotation speed of the internal combustion engine when the sensed rotation speed difference (ΔN) sensed by the rotation sensing means (80,84,8h) becomes equal to or smaller than a predetermined value in a case where the lockup clutch is switched through a slip engagement state to a full engagement state, and a first engagement control section configured to perform a first engagement control to bring the engagement state of the lockup clutch to the full engagement state after the rotation of the internal combustion engine is increased with the engagement state of the lockup clutch in the slip engagement state while a torque transmission capacity of the lockup clutch is increased when the lockup clutch is switched through the slip engagement state to the full engagement state by an ON operation of an accelerator of the vehicle, characterized in that the control means (7,8) is configured such that the transmission gear ratio control is prohibited during the first engagement control by the first engagement control section.