To examine the insulating mechanism of a novel 5d-electron system Sr 2IrO4, we study the ground state properties of a three-orbital Hubbard model using a variational Monte Carlo method. We find that the insulating state in the ground state phase diagram shows crossover behavior from a weakly-correlated to a strongly-correlated antiferromagnetic state. This crossover is characterized by the different mechanisms of the insulating state, i.e., changing from an interaction-energy driven insulator to a band-energy driven insulator with increasing the interaction. We discuss that Sr 2IrO4 is located around this crossover region and displays an anomalous behavior.
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