The transmission controller is configured to, when the received range position signal indicates that the range position is switched from a travelling range to a neutral range during travelling, output an instruction to disengage all of friction elements that are in an engaged state in the travelling range, output an instruction to at least one friction element before the instruction to disengage all of the friction elements that are in the engaged state in the travelling range is output, confirm a change in a rotation speed of a transmission input shaft after the instruction to disengage is output to the at least one friction element, and when there is no change in the rotation speed, diagnose that the friction element instructed to be disengaged is in an erroneously engaged state in which the friction element is not able to be disengaged.