The present invention enhances the reliability of wire-bonding strength by reducing a variation in the entire transformation amount of a wire. At point P1, a bonding wedge 21 abuts the wire 71, whereby a load is exerted on the wire 71. The wire 71 transforms by an amount of transformation A, and the transformation stops at point P2. The transformation amount A varies greatly. At point P3 (T1), ultrasonic wave vibration is exerted and the transformation of the wire 71 is restarted. At this stage, a variation in the transformation amount A is absorbed by an amount of transformation B and therefore a variation in the transformation amount A+B becomes small. At point P4 (T1+T2), the transformation amount A+B is maintained substantially constant. At this point P4, the transformation amount of the wire 71 is set to 0 and the measurement of the transformation amount of the wire 71 is started. And until the transformation amount of the wire 71 reaches an amount of transformation C (point P5), a load and ultrasonic wave vibration are given.