In a driving assistance device for a vehicle provided with an automatic driving mode and a manual driving mode, a standby member is provided to allow a driver seated at a seat to rest an arm. A sensor senses that a hand of a driver is placed at a position where the driver can reach the sensor while the driver rests the arm on the standby member. The sensor senses that a finger of the driver is in contact with the sensor. The sensor is provided in an arm rest.