1. A device for detecting an azimuth viewed from the device which is present in a field, the device comprising: a transmission array antenna having M-piece transmission antenna elements (M is an integer equal to or larger than 2) arrayed along an array axis set in advance and a receiving array antenna having N-piece receiving antenna elements (N is an integer equal to or larger than 2) arrayed along the array axis, wherein N×M-piece channels for transmission and reception are provided by combinations of each of the transmission antenna elements and each of the receiving antenna elements, wherein the transmission antenna elements are arrayed according to a first interval and the receiving antenna elements are arrayed according to a second interval, the first and second intervals being set such that an azimuthal difference between a main lobe and a grating lobe of a search wave transmitted from the transmission array antenna is equal to or larger than a detection range, the detection range being an azimuthal width within which the device is capable of detecting the azimuth and being set to either i) an amount which is a positive integral multiple of a unit azimuthal width through which the reception signals from mutually adjacent ones of the receiving antenna elements are given a phase difference of 360 degrees or ii) an amount which is narrower than the azimuthal width through which the reception signals from mutually adjacent ones of the receiving antenna elements are given the phase difference of 360 degrees; means for repeatedly selecting a channel group among the channels, the selected channel group being either a first type of channel group or a second type of channel group, wherein the first type of channel group is provided as M-piece channel groups each consisting of N-piece channels, each of the M-piece channel groups being provided corresponding to the same M-piece transmission antenna elements, and the second type of channel group is provided as N-piece channel groups each consisting of M-piece channels, each of the N-piece channel groups being provided corresponding to the same N-piece reception antenna elements, means for acquiring a reception signal for the channels by transmitting the search wave and receiving a reflected search wave, every channel group selected repeatedly, through each of channels belonging to the channel group selected by the selecting means; means for performing a first spatial frequency analysis with the acquired reception signals for all the channels, every channel group which is either the second type of channel group or the first type of channel group which has not been selected by the selecting means; means for performing a second spatial frequency analysis with the same frequency bins of either each of the N-piece channel groups or each of the M-piece channel groups, the frequency bins resulting from the first spatial frequency analysis; and means for determining an azimuth of a target that has reflected the search signal in adherence to analysis results from the second spatial frequency analysis.