A warning device selects, as a first candidate, an obstacle whose collision time period is equal to or shorter than a threshold time period and is the shortest among at least one obstacle which is present either in a front area or in a side area. The warning device selects, as a second candidate, the obstacle whose collision time period is equal to or shorter than a threshold time period and is the shortest among at least one obstacle which is present in the side area. The warning device displays a warning screen which guides driver's eyes to a direction of one candidate with the shortest collision time period from the first candidate and the second candidate.