This disclosure provides a new method for polarizing an electromagnetic wave having a frequency of not less than 0.1 THz and not more than 0.8 THz using a polarizer. The method comprises: a step (a) of preparing the polarizer; wherein the polarizer comprises a sapphire single crystalline layer and a CaxCoO2 crystalline layer, the CaxCoO2 crystalline layer is stacked on the sapphire single crystalline layer, a surface of the CaxCoO2 crystalline layer has a (010) surface orientation, and the CaxCoO2 crystalline layer has a thickness of not less than 2 micrometers and not more than 20 micrometers; and a step (b) of irradiating the polarizer with the electromagnetic wave having a frequency of not less than 0.1 THz and not more than 0.8 THz to output an output wave having only a component parallel to a c-axis direction of the sapphire single crystalline layer.