Pressure phase diagram of the antiferromagnet UIn3 was constructed from the electrical resistivity measurement under high pressures up to 9 GPa. A Néel temperature increases monotonically with increasing pressure from 88 K at ambient pressure to 127 K at 9 GPa. We observed an additional resistive anomaly at $T^{*} = 21$ K under 1.4 GPa. This anomaly brings about an increase of the residual resistivity. $T^{*}$ decreases gradually with increasing pressure and disappears around 8 GPa.