  A new formula for the gravitational potential of flattened systems is
proposed. It is a modification of the Miyamoto-Nagai potential and should be
applied to very flattened systems, exponential discs as a typical example. The
resulting rotation curve agrees sufficiently well with that obtained by using
special functions and the total masses remain the same. The functions contained
in the new term can improve the agreement for the rotation curve and also
reduce the effect of negative density values which appear off the midplane.
