To accurately obtain the radiation efficiency of an optionally-shaped antenna by using an oval sphere coupler.

An antenna to be measured 1 is arranged near a focal point F1 of an oval sphere closed space 12 obtained by rotating an ellipse centered around the axis passing through the 2 focal points F1, F2 of the ellipse, and a receiving antenna 15 is arranged near the focal point F2. An excitation signal is supplied to the antenna to be measured 1 and an output signal of the receiving antenna 15 is output from the closed space 12. While moving at least one of the antenna to be measured 1 and the receiving antenna 15 along the axis passing through the 2 focal points so that the mutual distance changes, at least one of a reflection ratio and a transmission ratio viewed from the side of the antenna to be measured and a pure resistive part of the antenna to be measured are measured. From the measured results, the pure resistive part Rm of the antenna to be measured 1 at the position where the least reflection is shown and the pure resistive part Ri of the antenna to be measured 1 at the position where the maximum reflection is shown are obtained. The radiation efficiency ηr of the antenna to be measured 1 is calculated by the following formula:ηr=1-(Ri/Rm).