Control device for power transmission device and control method for vehicle

A control device for a power transmission device, the power transmission device being mounted on a vehicle, the power transmission device including an automatic transmission and a sub transmission,

• the automatic transmission including a plurality of frictional engagement devices,
• the sub transmission being disposed in a power transmission path between the automatic transmission and driving wheels of the vehicle, the sub transmission being configured to change a rotation speed of an output shaft of the automatic transmission, the sub transmission including a high-speed engagement element and a low-speed engagement element, gear stages of the sub transmission including a high-speed gear stage and a low-speed gear stage,
• the high-speed engagement element being configured to be engaged when the high-speed gear stage is established,
• the low-speed engagement element being configured to be engaged when the low-speed gear stage is established,
• the control device comprising:
• an electronic control unit configured to switch the gear stage of the sub transmission,
• when the electronic control unit switches the gear stage of the sub transmission, the electronic control unit being configured to control one of the high-speed engagement element and the low-speed engagement element under engagement to be disengaged and configured to control the other one of the high-speed engagement element and the low-speed engagement element to be engaged via a power transmission cutoff state,
• the electronic control unit being configured to perform a rotation speed decrease control of decreasing the rotation speed of the output shaft of the automatic transmission,
• the electronic control unit being configured to perform the rotation speed decrease control in which at least one frictional engagement device of the plurality of frictional engagement devices is controlled to be one of an engaged state and a semi-engaged state at one of a time point before switching of the gear stage of the sub transmission is started and a time point at which the switching is started,
• the electronic control unit being configured to end the rotation speed decrease control during switching of the gear stage of the sub transmission, and
• the electronic control unit being configured to switch the sub transmission from the power transmission cutoff state to a state in which one of the high-speed engagement element and the low-speed engagement element that are engaged after the gear stage of the sub transmission is switched is engaged, during disengagement of the at least one frictional engagement device.