1. A method of calibrating an optical axis of a sensor installed on a hand of an arm of a robot by detecting a teaching tool placed at a semiconductor wafer placing position of a storage container or a carrying device by the sensor to teach the placing position of the semiconductor wafer to the robot, the method, comprising the steps of: placing the teaching tool at a specified position with the robot; predicting a position of the teaching tool by detecting the teaching tool with the sensor to obtain a predicted value; obtaining a difference between the position of the teaching tool and the predicted value; and performing calibration of the optical axis using the difference.