Driving assistance device and driving assistance method

Disclosed is a driving assistance device whereby a necessity for driving assistance relating to collision avoidance is appropriately managed, and an implementation of an unnecessary driving assistance is efficaciously alleviated. A driving assistance device carries out a collision avoidance assistance between a vehicle (10) and a vehicle ahead (60) which is present in the direction of progress of the vehicle (10), on the basis of a collision time Ta which is a time required for the vehicle (10) to collide with the vehicle ahead (60). A transverse change quantity computation unit (23) detects a relative transverse velocity Vy, which is a quantity of a relative change over time between the vehicle (10) and the vehicle ahead (60). A storage unit (40) stores a steering time T1, which is a time required for the vehicle (10) to steer to avoid the vehicle ahead (60). The storage unit (40) stores an implementation threshold TH1, which is a threshold for determining, on the basis of the relative transverse velocity Vy, an implementation of driving assistance. When the relative transverse velocity Vy, at a time when the collision time Ta is greater than or equal to the steering time T1, is greater than or equal to the implementation threshold TH1, an assistance management unit (50) alleviates the implementation of the driving assistance.