Magnetospheric convective system proposed by Axford and Hines is revised and made compatible with the view, based on the recently obtained geomagnetic data, that (1) the convective motion induced by the solar wind would penetrate to the innermost part of the magnetosphere, if it were not for the superposed effect of the earth's rotation, and the fact that (2) the earth's magnetosphere has an essentially open tail. Magnetic lines of force in the magnetosphere are then found separated into two groups: those that travel across the tail during the convective motion and those that are never transported to the tail. On field lines of the former group, the plasma density would be less than the value expected on the basis of the equilibrium theory, since the plasma along these field lines can escape to the outer space while the field line travels across the open magnetospheric tail and since the rate of plasma replenishment from lower levels is low. On field lines of the latter group, plasma escape is always prevented by closed field lines, so that the diffusive equilibrium would prevail. Hence at the boundary between these two groups of field lines, the plasma density is expected to show discontinuity. This boundary surface is suggested to be the plasmapause, and various observed features of the plasmapause are explained.