1. An object information acquiring apparatus, comprising: a plurality of receiving elements which receive acoustic waves emitted from an object and convert the acoustic waves into received signals; a delay unit which matches phases of the received signals; 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 the 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 all input data relating to one complex covariance matrix.