PURPOSE: To allow measurement of tensile force at a practical operating speed without requiring excessive draw out of wire material by providing an accumulator comprising a combination of a fixed wheel and a moving wheel.

CONSTITUTION: A wire rod 110 is drawn out from a bobbin 100 and entrained about a fixed wheel 5 and a moving wheel 6 in an accumulator 3 with the interval between the wheels 5, 6 being set at minimum value. Leading end of the wire rod 110 is then fixed to a balance 15 secured to a casing 2 Subsequently, a motor 13 is rotated to wind the wire 11 around a bobbin 12 and to move the wheel 6 in the direction departing from the wheel 5. Consequently, the wire rod 110 is run through movement of the wheel 6 and drawn out from the bobbin 100. In this regard, a force identical to that for drawing out the wire rod 110 from the bobbin 100 is imparted to the balance 15 thus allowing measurement of drawing tensile force on the balance 15. This constitution ensures a draw out distance without shifting the balance 15.