A calibration system for an excavator includes the excavator having an upper structure swingably attached to a chassis, a working machine having a boom, an arm, and a working tool, and a current position computation unit configured to compute a current position of a working point of the working tool; a calibration device configured to calibrate parameters based on parameters indicating dimensions and rotation angles of the boom, the arm, and the working tool; an external measurement device that measures a position of the working point; and an inclination information detection device that detects inclination information of the excavator in an anteroposterior direction. The calibration device corrects positions of the working point measured by the external measurement device, based on the inclination information of the excavator in the anteroposterior direction, and computes calibrated values of the parameters based on coordinates of the working point at corrected positions.