An inkjet printhead substrate includes: a pair of individual conductive layers configured to supply electrical power to a heat generation element; a first common conductive layer configured to be connected to one of the pair of individual conductive layers; a second common conductive layer configured to be connected to the other of the pair of individual conductive layers; and an isolation layer configured to be provided between the one of the pair of individual conductive layers and the first common conductive layer, and between the other of the pair of individual conductive layers and the second common conductive layer, wherein the isolation layer is formed from a conductive material which has a lower solubility in ink than a material used for the pairs of individual conductive layers, the first common conductive layer and the second common conductive layer.