A surface potential distribution measuring device that measures a surface potential of at least one electric field reduction system applied to a stator coil end which is an end portion of a stator coil of a rotating electrical machine, the device comprising: a laser configured to emit laser light; a Pockels crystal, extending to a longitudinal direction along the direction of the laser light, configured to receive, at its first end surface, the laser light emitted from the laser; a mirror having a mirror surface at a second end surface of the Pockels crystal and configured to reflect the laser light incident from the first end surface of the Pockels crystal in a direction opposite to an incident direction of the laser light; a light detector having a band following a high-frequency component of a pulse voltage and configured to receive the laser light reflected by the mirror and detecting, as a light intensity of the laser light, a detection light intensity corresponding to an output voltage which is a potential difference between the first end surface of the Pockels crystal and the second end surface thereof; a voltage calibration database configured to store, when a plurality of mutually different input voltages are applied to a rear surface of the mirror in voltage calibration processing to be performed before a test, input-to-output voltage characteristics representing a relationship between the different input voltages and the output voltages of the Pockels crystal obtained when the different input voltages are applied to the rear surface of the mirror; and a computing section configured to set, when a part of a surface of the electric field reduction system is set at a rear side of the mirror as a test location in surface potential measurement processing to be performed during the test, the output voltage of the Pockels crystal obtained when a voltage is applied to the stator coil as an output voltage at test time and identifying an input voltage corresponding to the output voltage at test time as the surface potential of the electric field reduction system based on the input-to-output voltage characteristics stored in the voltage calibration database, wherein the first end surface of the Pockels crystal is grounded. a laser configured to emit laser light; a Pockels crystal, extending to a longitudinal direction along the direction of the laser light, configured to receive, at its first end surface, the laser light emitted from the laser; a mirror having a mirror surface at a second end surface of the Pockels crystal and configured to reflect the laser light incident from the first end surface of the Pockels crystal in a direction opposite to an incident direction of the laser light; a light detector having a band following a high-frequency component of a pulse voltage and configured to receive the laser light reflected by the mirror and detecting, as a light intensity of the laser light, a detection light intensity corresponding to an output voltage which is a potential difference between the first end surface of the Pockels crystal and the second end surface thereof; a voltage calibration database configured to store, when a plurality of mutually different input voltages are applied to a rear surface of the mirror in voltage calibration processing to be performed before a test, input-to-output voltage characteristics representing a relationship between the different input voltages and the output voltages of the Pockels crystal obtained when the different input voltages are applied to the rear surface of the mirror; and a computing section configured to set, when a part of a surface of the electric field reduction system is set at a rear side of the mirror as a test location in surface potential measurement processing to be performed during the test, the output voltage of the Pockels crystal obtained when a voltage is applied to the stator coil as an output voltage at test time and identifying an input voltage corresponding to the output voltage at test time as the surface potential of the electric field reduction system based on the input-to-output voltage characteristics stored in the voltage calibration database, wherein the first end surface of the Pockels crystal is grounded.