The present invention relates to a tri-axial compression tester including a tri-axial chamber having an internal space wherein a test piece specimen is installed and a lateral pressure applying unit applying a preset target lateral pressure to the test piece specimen in a transverse direction by supplying a pressure fluid to the internal space. The lateral pressure applying unit includes a pressure fluid supplying unit supplying the pressure fluid to the internal space of the tri-axial chamber; a lateral pressure sensor measuring an actual lateral pressure applied to the test piece specimen in a transverse direction by the pressure fluid supplied from the pressure fluid supplying unit; and a lateral pressure controlling unit receiving the actual lateral pressure from the lateral pressure sensor, comparing the actual lateral pressure with the target lateral pressure, and making the actual lateral pressure identical to the target lateral pressure by proportional integral derivative (PID) controlling the pressure fluid supplying unit to remove the difference between the target lateral pressure and the actual lateral pressure if the pressures of the target lateral pressure and the actual lateral pressure are different. Accordingly, the present invention is capable of automatically making the actual lateral pressure actually applied to the test piece specimen identical to the target lateral pressure desired to apply to the test piece specimen by PID-controlling the fluid supplying unit capable of supplying the pressure fluid to the internal space of the tri-axial chamber installed in the test piece specimen, thereby accurately testing material properties of the test piece specimen and improving user convenience for testing.
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