An inkjet printhead substrate includes a base plate, a heat storage layer placed on the base plate, a heat-generating resistive layer which is placed on the heat storage layer and which includes an electrothermal transducing portion, a wiring layer electrically connected to the heat-generating resistive layer, and an insulating protective layer covering the heat-generating resistive layer and the wiring layer. The heat storage layer includes a porous cyclic silazane film formed by a vapor phase process.