A control unit (200) executes fail-safe control that forcibly releases a lockup clutch (15) by activating a fail-safe valve (112), abnormality diagnosis control that determines whether a solenoid valve (DSU) is suffering from a solenoid ON abnormality when the vehicle has started with the fail-safe control executed, and reproduction control that simulatively reproduces, when the vehicle has started without the fail-safe control executed, a condition coinciding with the condition for allowing execution of the abnormality diagnosis control.