On the other hand, when a substance, such as a raindrop or dirt, adheres to the front glass in front of one of the main camera 2 a and the sub-camera 2 b, for example, when dirt adheres to the front glass in front of the sub-camera 2 b and an estimated region RCest is set in a portion of the comparative image TC corresponding to the dirt, an average p2 ij ave of luminances P2 ij in the estimated r