This paper describes the estimation method of the sound power radiating from a four side fixed rectangular plate acoustically excited in a diffused sound field. First, the forced vibration of the plate excited by a plane wave in a free sound field has been solved. In the calculation, the distribution of the plane angles of modal exciting forces is taken into consideration precisely. Next, the stochastic model composed of random plane wave elements is assumed on the diffused sound field. The modal exciting forces and the forced vibration displacement are derived based on the ergodic process. Using the obtained forces and displacement, the equations for calculating the frequency spectra of the parameters for estimation are derived. The estimated frequency responses of radiating sound power coincided well with the measured values.