PURPOSE: To eliminate cutting work of undercut thus to prevent fluctuation of fuel delivery characteristic, by forming a guide hole and a valve seat and fuel delivery port communicating with the guide hole in a valve seat mold while employing a linear guide hole applied with varnishing.

CONSTITUTION: A guide hole 9 of valve seat mold 8 is made linear having no undercut at the varnished leading edge where the surface of under hole 9' for guide hole 9 made through the material of valve seat mold 8 is varnished by means of a varnishing tool 17 thus to eliminate cutting work of undercut for preventing polishing burr and to produce a fuel injection vlave where the guide hole 9 is formed linearly without undercut. Consequently, porductionability is improved while pressure loss due to exixtence of undercut is prevented resulting in a stable injection valve where the fuel delivery characteristic is not fluctuated.