Driving assistance device and driving assistance system

The purpose of the present invention is to enable lane departure warning and control even in an environment in which there is no lane line and the precision of GNSS is low, a road having multiple lanes, or an intersection. A driving assistance device is provided with: a recording unit that records a plurality of past travel positions of a vehicle and position estimation information relating to the past travel positions; a relative position estimation unit that estimates the relative position of the vehicle on the basis of the output of a sensor that detects the surroundings of the vehicle, adds a history of the relative position to a past travel position, and adds the output of the sensor used in the estimation of the relative position to the position estimation information; a position estimation unit that estimates the position of the vehicle with respect to the past travel position on the basis of the output of the sensor and the position estimation information from the recording unit, and adds the history