The acoustic radiation power of planar phased array sound sources consisting of Tonpilz-type transducers was measured. The correct measurements of active radiation power can be obtained in a manner independent of the distance between the measurement and radiation surfaces, provided that the measurement surface sufficiently covers the radiation surface. Furthermore, it has been experimentally demonstrated that the measured reactive radiation power varies as a function of the distance from the measurement surface to the radiation surface, and that correct values can be obtained if measurements are performed as close to the radiation surface as possible. Fluctuation of the radiation power of the phased array is caused mainly by radiation impedance between two adjacent transducers.