To make appropriate announcement, when a preceding vehicle can not be detected due to the influence of sunlight.

A sunlight obstacle condition determining part 213 calculates whether a straight line connecting a laser radar 11 to the sun is in the range of an incident angle of a light-receiving part 222 of the laser radar 11, from the position of the sun at a calendar and a time calculated by a sun position calculating part 212, the own vehicle position calculated by an own vehicle position detecting part 216, the own vehicle attitude calculated by an own vehicle attitude detecting part 217, and the illuminance outside the vehicle measured by an illumination sensor 203. The sunlight obstacle condition determining part 213 determines the threshold for determining a sunlight obstacle and an elapsed time, until the determination is reached, corresponding to the positional relation between the laser radar 11 and the sun calculated by the sun position calculating part 212.