An inkjet printhead substrate includes: a heat generating element configured to generate energy for ejecting ink; an electric wire electrically connecting the heat generating element and an electrode lead provided on a flexible film wiring substrate; a protecting film configured to protect the electric wire; an electrode pad to which the electrode lead is connected, the electrode pad being formed by providing an opening in the protecting film at a position above the electric wire; a region to which a sealing resin configured to protect an electrically connected portion of the electrode pad and the electrode lead is to be applied; and an ink-detecting electrode composed of a metal wire and formed at the region to which the sealing resin is to be applied. The metal wire has a smaller width than an opening provided in the protecting film from which the metal wire is exposed.