1. An apparatus, comprising: one or more memories; and circuitry that, in operation, performs operations, including: calculating a first correlation matrix, which corresponds to observed signals, which are acoustic signals obtained by a microphone array including two or more separately arranged microphones, specifying a non-scan range, which indicates a direction range within which a target sound source is not to be detected, estimating a second correlation matrix, which corresponds to an acoustic signal from a sound source within the non-scan range, calculating a third correlation matrix, which corresponds to the target sound source within a scan range, which indicates a direction range within which the target sound source is to be detected, by removing the second correlation matrix from the first correlation matrix, and calculating a first spatial spectrum on the basis of the third correlation matrix as a localization result, wherein, in the estimating, the second correlation matrix is estimated on the basis of direction vectors obtained from the direction range of the non-scan range and a second spatial spectrum, which is a localization result immediately before the first spatial spectrum is calculated.