To provide a selection assistant device of an automatic transmission capable of expanding layout flexibility by downsizing a selection lever and also capable of obtaining a characteristic of selection lever control force according to a demand while securing reliability by a mechanical connection between the selection lever and a range position switch device.

An electric motor 15 assisting and driving the selection lever 2 is provided, and the fixed end 218 is provided close to the end of P range of the P range-L range of the selection lever 2. When the selection lever 2 is in the P range, the electric motor 15 is driven at a driving current value generating predetermined torque, the selection lever 2 is pressed against the fixed end 218, reaction force is given to the selection lever 2, and an anomaly of a torque sensor 21 is diagnosed based on whether the torque sensor 21 outputs the predetermined torque or not.