A process for producing a substrate for a liquid ejection head, including a step of forming a liquid supply port passing through a silicon substrate by dry etching, the step being a step of sequentially repeating the steps of (1) forming an etching protection film on the silicon substrate, (2) removing a bottom portion of the etching protection film, and (3) etching the silicon substrate, wherein a sheath formed in the step (2) is thicker than a sheath formed in the step (3).