The thermocline oscillation observed in the stratified water mass, such as, deep lakes, reservoirs and land-locked bays, are sometimes generated by the resurging effects after the stop of the wind set-up motion. The oscillation characteristics of the density interface become much complex in the case of complicated geometry, e. g. reservoirs with several branches, bays composed of many inlets, etc. The authors developed the theory for the, interfacial seiche motion in the two-layered and multi-composed water mass by applying the theory of the surface seiche. Additionally, a laboratory experiment was performed by using the model tank composed of four channel elements with rectangular cross sections. Satisfactory agreements between the theory and experiment were obtained.