A theoretical problem of the penetration of a western boundary current, driven by a cross-stream sea level difference, onto a shallow basin to the west of the current is put forth to account for the penetration of the Kuroshio onto the East China Sea (ECS) shelf and formation of the Tsushima Warm Current. Different from previous studies, this study assumes that the change of bottom depth is the primary factor determining the penetration. For a given depth of the shallow basin, the penetration depends on two external parameters: speed of the western boundary current, V 0, and surface elevation of the current at the edge of the shallow basin, Z 0. Two penetration modes emerge, depending on the magnitude of the inertia of penetration. For sufficiently large inertia, which is favored by large Z 0 and hindered by large V 0, penetration occurs in the form of a free jet, mode "J." Otherwise, it is in the form of a coastal current, mode "C." Volume transport of the penetration is maximized in mode "J." In general, volume transport, width, and speed of the penetrating current increase with Z 0, although the latter two are slightly more dependent on V 0 in mode "C." The model considered in this study predicts that the penetration of the Kuroshio onto the ECS shelf is of mode "J," as indicated by observations. The volume transport of the penetration is somewhat overestimated, possibly because bottom friction is ignored in this model. Although this model is highly idealized, it allows one to gain insight into the behavior of the Kuroshio penetrating onto the shallow ECS shelf.