The pressure dependence of the etch rate in an Al etching reactor has been studied with the goal of developing a method of numerically simulating the etching process. Experimentally, the etch rate was measured at pressures between 3 and 100 mTorr and reached a maximum at about 10 mTorr, whereas the calculated etch rate, which took the etching reaction between Al and Cl-2 on an Al surface into account, increased with pressure. To explain the experimental data, the residence time tau(0) Of the inhibitor adsorbed on the Al surface and time-averaged surface coverage were included in the calculations. In the low-pressure region below 10 mTorr, the etch rate increased with an increase in pressure, due to the increased Cl-2 flux on the Al surface. On the other hand, in the high-pressure region over 10 mTorr, the etch rate decreased when the effect of the inhibitor became stronger; this was explained by developing an improved model. Consideration of the inhibitor adsorbed on the Al surface is important, where the increase of tau(0) with pressure is necessary to precisely simulate the correlation between the etch rate and pressure.