[en] The intrusion behavior of a lighter fluid into a flowing heavier fluid is investigated in the condition of the density stratified flow. The relationship of the intrusion velocity and the layer thickness are described, in terms of the upstream Froude number and the effect of the frictional shear stress. The intrusion velocity can be expressed well by the present model with use of the appropriates approximation of the friction coefficient. The relation between the intrusion velocity and layer thickness was determined by the criterion of the interfacial wave instability. The data showed good agreement with existing correlation as Kordyban and Ranov and Mishima and Ishii. (author)