A three-dimensional surface potential distribution measurement apparatus to measure a surface potential of an electric field relaxation system which is formed along a longitudinal direction of a test object simulating a stator coil end of a rotating electrical machine, the apparatus comprising: a laser light source configured to emit laser light; a Pockels crystal configured to receive, at its first end face, the laser light emitted from the laser light source; a mirror having a mirror surface positioned at a second end face of the Pockels crystal that is opposed to the first end face, the mirror being configured to reflect the laser light from the one end face of the Pockels crystal in a direction opposite to the incident direction of the laser light; a light detector having a band following a high-frequency component of an inverter pulse voltage, the light detector being configured to receive the laser light reflected by the mirror, and to detect a light intensity of the laser light corresponding to an output voltage which is a potential difference between the first end face of the Pockels crystal and the second end face thereof; said measurement apparatus is characterised by: a support structure configured to support the laser light source, the Pockels crystal, the mirror, and the light detector while maintaining a relative positional relationship thereamong; a movement driver (32a, 33a, 34a) capable of three-dimensionally moving the support structure; a rotary driver configured to support the test object, the rotary driver being capable of rotating the test object about an axis extending in a longitudinal direction of the test object in both clockwise and counterclockwise directions; and a drive controller configured to control the movement driver (32a, 33a, 34a) and the rotary driver, wherein the drive controller coordinates a driving operation by the movement driver (32a, 33a, 34a) and a driving operation by the rotary driver while maintaining a gap between the second end face of the Pockels crystal and a surface of the test object at a predetermined value such that the second end face of the Pockels crystal approaches the surfaces of the electric field relaxation system on the test object. a laser light source configured to emit laser light; a Pockels crystal configured to receive, at its first end face, the laser light emitted from the laser light source; a mirror having a mirror surface positioned at a second end face of the Pockels crystal that is opposed to the first end face, the mirror being configured to reflect the laser light from the one end face of the Pockels crystal in a direction opposite to the incident direction of the laser light; a light detector having a band following a high-frequency component of an inverter pulse voltage, the light detector being configured to receive the laser light reflected by the mirror, and to detect a light intensity of the laser light corresponding to an output voltage which is a potential difference between the first end face of the Pockels crystal and the second end face thereof; said measurement apparatus is characterised by: a support structure configured to support the laser light source, the Pockels crystal, the mirror, and the light detector while maintaining a relative positional relationship thereamong; a movement driver (32a, 33a, 34a) capable of three-dimensionally moving the support structure; a rotary driver configured to support the test object, the rotary driver being capable of rotating the test object about an axis extending in a longitudinal direction of the test object in both clockwise and counterclockwise directions; and a drive controller configured to control the movement driver (32a, 33a, 34a) and the rotary driver, wherein the drive controller coordinates a driving operation by the movement driver (32a, 33a, 34a) and a driving operation by the rotary driver while maintaining a gap between the second end face of the Pockels crystal and a surface of the test object at a predetermined value such that the second end face of the Pockels crystal approaches the surfaces of the electric field relaxation system on the test object. a support structure configured to support the laser light source, the Pockels crystal, the mirror, and the light detector while maintaining a relative positional relationship thereamong; a movement driver (32a, 33a, 34a) capable of three-dimensionally moving the support structure; a rotary driver configured to support the test object, the rotary driver being capable of rotating the test object about an axis extending in a longitudinal direction of the test object in both clockwise and counterclockwise directions; and a drive controller configured to control the movement driver (32a, 33a, 34a) and the rotary driver, wherein the drive controller coordinates a driving operation by the movement driver (32a, 33a, 34a) and a driving operation by the rotary driver while maintaining a gap between the second end face of the Pockels crystal and a surface of the test object at a predetermined value such that the second end face of the Pockels crystal approaches the surfaces of the electric field relaxation system on the test object.