A new spectropolarimeter is developed at the Domeless Solar Telescope
(DST) in Hida Observatory. It consists of a rotating waveplate, Wollaston prisms, and a high-dispersion spectrograph which is vertically installed at the focus of the DST. In order to realize a high-precision measurement, it is inevitable to compensate the instrumental polarization due to the DST. We observed the quiet region of the Sun, which is considered to be highly unpolarized, with and without a sheet linear polarizer or circular polarizer set at the entrance window of the telescope. The theoretical model which represents the total instrumental polarization of the DST with some characteristic parameters was calculated and compared with the observation. The model that two flat mirrors have different properties can explain the observation in 0.5% accuracy for the unpolarized light, and in 7% for the polarized light.