A manufacturing method of a substrate for display panel comprises a step of forming an organic insulating film on a light transmissive substrate and a step of patterning the organic insulating film to form a first opening and a second opening smaller than the first opening formed as through holes therein. In particular, this patterning step includes a lithography process of selectively exposing the organic insulating film to light with the use of a photomask and removing the light-exposed portions, the photomask being so set that the average light transmittance is lower at a light transmissive area for the first opening than at a light transmissive area for the second opening.