The attenuation characteristics of a multilayer metal-clad optical guide, which is suitable for a mode filter or electrooptic devices, is investigated by the exact theory and the analytical approximation based on a perturbation technique. By using this approximation the dependences of the ohmic loss on the various waveguide parameters and the condition for the absorption peak of the TM mode are derived in closed form. Some remarks concerning the waveguide material and dimension for the design of the mode filter are also presented. The insertion loss at the abrupt junction between a normal (dielectric-clad) optical guide region and a metal-clad optical guide region is treated. Also the transformation of optical guided modes in the two kinds of tapered structures between the above two regions is examined.