To calibrate an overall precision of a speed measuring instrument by using only a gravitational acceleration.

The speed measuring instrument B is arranged in an instrument calibration posture with a frame rotated by 90° to make a line segment connecting the first and second respective sensors K1, K2 constituting the speed measuring instrument B laid along a vertical direction, and is fallen naturally using a longitudinally long specified speed slider S1, in a speed measuring posture of the speed measuring instrument B, a falling-down theoretical calculated time T1 until the second detected part 36 is detected by the second sensor K2 after the first detected part 35 is detected by the first sensor K1 is calculated theoretically, the plurality of respective first to Nth specified speed sliders S1-Sn different in the falling-down theoretical calculated times is prepared preliminarily, the first to Nth specified speed sliders S1-Sn are fallen down naturally, and each measured time measured practically by the speed measuring instrument of a calibration object is compared with the each falling-down theoretical calculated time to calibrate the precision of the speed measuring instrument that is precision of the calibration object.