We report a Monte Carlo study of the hysteretic behavior of the site-diluted Ising antiferromagnet on a simple-cubic lattice. States found by reducing the magnetic field from an initial value in the paramagnetic phase show no long-range antiferromagnetic order, whereas states found by increasing the magnetic field from an initially ordered state do show long-range order until the field is increased to a critical value. These ‘‘increasing-field’’ states are presumably nearly representative of true thermodynamic equilibrium. This hysteretic behavior is due to metastable interfaces which are formed when the field is decreased from the paramagnetic phase. To see the physical origin of the formation of the metastable interfaces, we do computer experiments for the dynamics of initially flat interfaces with and without uniform applied fields. From this result, we show that metastable interfaces are formed, because even a small uniform applied field roughens the interface.