PURPOSE: To autonomously calibrate errors to a calibration straight line indicating an input/output relationship of a sensor or the like accurately by its own output using neither a standard measuring apparatus nor an auxiliary input estimation apparatus.

CONSTITUTION: In the system to correct linear errors of a PSD (semiconductor position detector) 1 which detects the position of the center of gravity of a light spot, two mirrors 4a and 4b which are given inclination a micrometer 2, are allowed to set the proportion of the inclination of both the mirrors to n:1 by adjusting the positions thereof on a cantilever beam 6. When the lengths of optical paths of the PSD1 from both the mirrors are made equal to each other, the positions of two light spots on the PSD shift at the proportion of n:1 as the inclination of the cantilever beam is varied. By lighting two light beams alternately, the reading of values of a reference axis of a calibration curve and an output axis thereof can be read. The beam serving as input on the reference side is changed in the direction of incidence with the rotation of a turntable 5, thereby, the light spots can be returned to the end point of a PSD1 measuring range.