A transmission ECU 12 determines whether or not the conditions for executing a neutral control operation are satisfied (step S11), and measures hydraulic oil temperature if it determines that the execution conditions are satisfied (step S12). Then, the transmission ECU 12 sets a target speed ratio for a torque converter 3 corresponding to the measured hydraulic oil temperature (step S13), and performs a neutral control operation to bring the speed ratio of the torque converter 3 equal to the target speed ratio (step S14).