Soft X-ray spectra have been measured using a pair of YB66(400) monochromator crystals at the double-crystal monochromator beamline BL7A of the UVSOR facility, where the wiggler radiation has a magnetic field of 4 T. Deformation of the YB66 crystal due to heat load from the synchrotron radiation is almost negligible. The photon flux is about 10(8) photons s(-1) (100 mA)(-1) in the energy region 1.2-2 keV and the energy resolution is 0.7 +/- 0.1 eV around hv = 1.5 keV. These results show that the YB66 crystal is suitable for use as a monochromator crystal. Its application to soft X-ray spectroscopy is discussed.