A reflected wave that a transmitted wave is reflected by targets including a stationary target and a moving target is received as receiving signals of at least two reception antennas. A phase difference between beat signals at respective peak frequencies, generated from a transmitting signal and the receiving signals, is calculated. Directions of the targets are calculated in such a manner that the calculated phase difference of the targets is stored in a storage area in advance, it is predicted whether peak frequencies of the plurality of targets overlap each other, a predicted phase difference of the stationary target at the time when the peak frequencies overlap each other is calculated on the basis of the stored phase difference, and a predicted phase difference of the moving target is calculated on the basis of the calculated phase difference and the predicted phase difference of the stationary target.