PURPOSE: To obtain a method, through which the substantial defectiveness of wire bonding is detected simply and positively, by upward erecting a defective bonding wire, exposed to an upward gas flow and detecting the erected defective bonding wire by a detecting means.

CONSTITUTION: Wires are bonded with pellets 2 bonded onto forward fed lead frames 1 in succession, the pellets 2 are exposed in an upward gas flow and a defective bonding wire 3 is erected upward, and such an erected defective bonding wire 3 is detected by a detecting means 13. An apparstus in which the terminals of two optical fibers of an optical fiber 14a, which is generally called a fiber sensor and a base end of which is connected to a light-emitting element, and an optical fiber 14b, a base end of which is connected to a photodetector, are directed and disposed in the detecting direction is used as said detecting means 13.