There is provided a driving assistance device for supporting driving of a vehicle equipped with the driving assistance device such that if a driver starts a driving-related operation in the course of self-driving, switching from self-driving to manual driving is performed. An acquiring unit configured to acquire conditions of the driver. A determining unit configured to determine to invalidate switching to manual driving in the case where the conditions of the driver acquired when the operation was started represents a non-waking state, and determine to continue the invalidation for a predetermined period if the driver changes from the non-waking state to a waking state when the invalidation is being performed.