A plurality of array antenna elements are divided in terms of a set constituted of an optionally selected L number of groups. The spaces between the array antenna elements are determined by obtaining a greatest common divisor of the set as a greatest common divisor of inter-group element spaces, the set having, as components, greatest common divisors of inter-element spaces of the individual L number of groups in the set, obtaining a greatest common divisor of inter-group element for every number L equal to or less than the maximum number of incoming waves by changing the number L of group components, and allowing a direction not to be caused in the radar scanning region by a number equal to or more than L+1, on the basis of the greatest common divisor of inter-group element spaces for every number L of group components, the direction being linearly dependent on an incoming wave corresponding to the greatest common divisor of inter-group element spaces.