1. An object information acquiring apparatus, comprising: a delay unit which matches phases of received signals output from a plurality of receiving elements that has received acoustic waves emitted from an object; a complex converter which converts the received signals output from said delay unit into complex signals; a complex covariance matrix calculator which periodically obtains a complex covariance matrix by using a complex signal group configured from a plurality of phase-matched complex signals; an eliminator which uses, as input data, at least either the complex signal group or matrix elements contained in the complex covariance matrix, and reduces a number of bits of the input data; and an electric power calculator which calculates a power value of each target position based on the complex covariance matrix, wherein said eliminator reduces the number of bits by performing common level conversion processing on a plurality of input data relating to one complex covariance matrix.