We have found an anomaly in magnetic field dependence of the de Haas-van Alphen (dHvA) frequency of ZrZn2. Pressure dependence of this anomaly suggests an existence of two distinct ferromagnetic states and then a "nested" phase diagram similar to that of UGe2. The Stoner exchange parameter and the exchange splitting energy are 400 and 66 meV, respectively, which are derived from comparing the field dependence of the dHvA frequency difference with the magnetization. The two parameters do not change in both the low and slightly higher average moment states, suggesting that the phase transition between these states is ascribed to the change in the difference of majority and minority spin carrier densities.
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