S.Kovac proved that a nef cone of a K3 surface coincides with the positive cone or can be cut out by the hyperplanes defined by (-2)-curve. We generalize this result to higher dimensional irreducible symplectic manifolds. As an application, we show that irreducible symplectic manifolds whose automorphism group contains an infinite cyclic group form dense subset of its moduli space if the second Betti number is greater than 6, which gives an approach to Abundance conjecture of irreducible symplectic manifolds.