This paper applies the principle of radar polarimetry to the synthetic aperture frequency modulated continuous wave radar. First, the principle of monochromatic wave radar polarimetry using scattering matrix and polarization ratio necessary for introducing polarimetric imaging is given. In order to accommodate this principle to a wideband radar, a scattering matrix must be introduced, because FM-CW radar utilizes a wideband signal. This paper points out that the polarimetric target reflection coefficient obtained by the synthetic aperture FM-CW radar works as the scattering matrix element. This replacement, i.e., polarimetric reflection coefficient = the scattering matrix element, was verified by an experiment based on the polarization ratio which maximizes and minimizes a target. A radar system operative in the microwave X-band was successfully applied to the polarimetric detection of a metallic pipe of different orientations, demonstrating the validity of FM-CW radar polarimetry, and indicating an establishment of full polarimetric radar system.