We did a beam test of the second model of ATF2 prototype cavity BPM. Before the installation, we did xy-isolation tuning of the cavity with tuners placed on the rim. The isolation of the two dipole modes was improved to be better than -50 dB, according to the network analyzer measurement. We checked sensitivity to the beam position (both 2-port and 1-port readout setups), and xy-isolation with a real beam. It was confirmed that xy-isolation was better than -40 dB.