Adsorption of CO on Cs-precovered Ru(001) with one-monolayer coverage (theta(Cs) = 0.33) has been investigated by use of electron energy loss spectroscopy (EELS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). The C-O stretching mode of the adsorbed CO does not appear in the specular mode of EELS but clearly appears in the off-specular mode at an unusually low frequency of 1245 cm(-1). This unusual adsorption state has been interpreted in terms of an inclined or a side-on adsorption geometry of CO and formation of a double-layer structure (Cs/Cs + CO/Ru(001)) where the CO dipole is screened by the second-layer Cs.