A target antenna 4A to be measured and a measuring antenna 5 are placed within an anechoic chamber 1. A first azimuth angle plane radiation pattern S21(R,θ,φ) is measured using a network analyzer 7 with an elevation angle φ of the target antenna 4A fixed to a first angle φ1. Next, a second azimuth angle plane radiation pattern S21(R,θ,φ) is measured using the network analyzer 7 with the elevation angle φ of the target antenna 4A fixed to a second angle φ2 different from the first angle φ1 by 90 degrees. A radiation efficiency of the target antenna 4A is measured by integrating in a spherical shape the azimuth angle plane radiation patterns S21(R,θ,φ) at two planes.