PURPOSE: To provide a method of manufacturing a substrate for mounting electronic components, whereby the burrs produced between an end face of a dielectric substrate and a dam bar can surely and easily be removed, and the quality of products as a substrate for mounting electronic components can also be assured.

CONSTITUTION: Release films 20 are stuck to both sides of an outer lead part 12 of each lead 11, then dielectric substrates 30 provided on both sides of a leadframe 10 and this leadframe 10 are integrated together. The dielectric substrate 30 positioned at a boundary between an inner lead part 13 and an outer lead part 12 is separated, and only the dielectric substrate 30 located on the outer lead 12 is removed. Burrs produced between an end surface of the dielectric substrate 30 and a dam bar 14 are removed together with the release film 20 by laser processing, and the remaining release films 20 are also eliminated.