A method for determining a reference angle of a probe around a rotation axis in a high-frequency test system including a controller, a vector network analyzer controlled by the controller, a stage having a flat mounting surface on which a device under test is mounted, a probe having a signal terminal and ground terminal that are able to send and receive signals with the vector network analyzer, the probe being able to change the state of contact of the tip of the signal terminal and tip of the ground terminal with the device under test by rotating with the rotation axis as the center, and a driving unit that is controlled by the controller and causes rotation of the probe around the rotation axis, comprising: the controller controlling the driving unit to change the angle of the probe around the rotation axis, thereby changing the state of contact of the tip of the signal terminal and tip of the ground terminal, with the conductor surface of the substrate that has a flat conductor surface mounted on the mounting surface of the stage, while controlling the vector network analyzer to output a high-frequency signal from the signal terminal to the conductor surface and receive a reflected signal using the probe to find the S-parameter, at different angles of the probe around the rotation axis; and the controller determining, based on a plurality of the S-parameters, the reference angle of the probe around the rotation axis at which the reference line formed connecting the tip of the signal terminal and tip of the ground terminal is parallel with the conductor surface. a controller, a vector network analyzer controlled by the controller, a stage having a flat mounting surface on which a device under test is mounted, a probe having a signal terminal and ground terminal that are able to send and receive signals with the vector network analyzer, the probe being able to change the state of contact of the tip of the signal terminal and tip of the ground terminal with the device under test by rotating with the rotation axis as the center, and a driving unit that is controlled by the controller and causes rotation of the probe around the rotation axis, comprising: the controller controlling the driving unit to change the angle of the probe around the rotation axis, thereby changing the state of contact of the tip of the signal terminal and tip of the ground terminal, with the conductor surface of the substrate that has a flat conductor surface mounted on the mounting surface of the stage, while controlling the vector network analyzer to output a high-frequency signal from the signal terminal to the conductor surface and receive a reflected signal using the probe to find the S-parameter, at different angles of the probe around the rotation axis; and the controller determining, based on a plurality of the S-parameters, the reference angle of the probe around the rotation axis at which the reference line formed connecting the tip of the signal terminal and tip of the ground terminal is parallel with the conductor surface. the controller controlling the driving unit to change the angle of the probe around the rotation axis, thereby changing the state of contact of the tip of the signal terminal and tip of the ground terminal, with the conductor surface of the substrate that has a flat conductor surface mounted on the mounting surface of the stage, while controlling the vector network analyzer to output a high-frequency signal from the signal terminal to the conductor surface and receive a reflected signal using the probe to find the S-parameter, at different angles of the probe around the rotation axis; and the controller determining, based on a plurality of the S-parameters, the reference angle of the probe around the rotation axis at which the reference line formed connecting the tip of the signal terminal and tip of the ground terminal is parallel with the conductor surface.