Hardness of bonding end portions of an external connection terminal to be bonded to circuit patterns of an insulating substrate which is not lower than 90 in Vickers hardness is disclosed. An ultrasonic welding tool is used. In the external connection terminal in which the bonding end portions are provided integrally with a bar, one of the bonding end portion located substantially in the lengthwise center of the bar is first bonded, and the other bonding end portions are bonded alternately in order toward either end. The hardness of the bonding end portions is increased so that strength of the ultrasonic welding portions is increased. Since the external connection terminal including the bonding end portions is bonded in such a manner that the bonding end portion located substantially in the center is first bonded and the other bonding end portions are then bonded in order of increasing distance substantially from the central bonding end portion, displacement of the bonding end portion in either end from its regular position can be suppressed to keep bonding strength high. In this manner, the bonding strength of the ultrasonic welding portions between the external connection terminal and the circuit patterns of the insulating substrate can be increased so that long-term reliability can be secured in a semiconductor device.