To highly accurately measure electromagnetic wave to be measured.

An electromagnetic wave measuring instrument for measuring electromagnetic wave to be measured which is transmitted by a measured object is equipped with a measurement antenna for receiving the electromagnetic wave to be measured, a reference antenna for transmitting or receiving reference electromagnetic wave having a known polarization direction, a rotary hold part for holding the measurement antenna rotatably relative to the reference antenna, and a measurement part for measuring the intensity of the reference electromagnetic wave transmitted by the reference antenna and received by the measurement antenna or the intensity of the reference electromagnetic wave transmitted by the measurement antenna and received by the reference antenna.