the stage s 8 following the stage s 7 or s 8 the motor is reversed to raise the holder means at the stage s 9 at the stage s 10 a sensor or switch not shown that has returned to the upper limit position detects the reset position and stops the motor in the afore described procedure the flow represented by the broken line route in fig5 is replaced with one indicated by a solid line and the tensile force during the reference holding time hereinafter referred to as 10 seconds with the reference load hereinafter referred to as 90 n is added for the measurement purpose this stage may be performed for example with the servo control of the motor in the following way if the stage s 2 is yes or u2267 90 n the motor is stopped at the stage s 4 and time counting is started at the stage s 5 cumulative counting is done for a unit time period e g one second when the cumulative count is less than 10 seconds whether the force of the load cell at the stage s 6 is at or above the reference value or not is determined if it is below the reference value the cycle returns to the stage s 1 where the motor is started again if it is at or above the reference.