Liquid discharge head substrate and head unit

A liquid discharge head substrate includes an external terminal, a diode, a first conductive layer, a second conductive layer, and a third conductive layer. The external terminal is configured to connect to an external. The first conductive layer is connected to the external terminal for causing an input current to flow from the external terminal, and the diode includes an anode and a cathode. The second conductive layer is connected to the first conductive layer and one electrode of the anode and cathode, and causes a surge current generated when a surge voltage is applied from the external terminal, to flow from the first conductive layer to the one electrode. The third conductive layer is connected to the other electrode of the anode and the cathode and passes the surge current flowing from the one electrode to the other electrode. The first conductive layer includes a part laminating with the second conductive layer sandwiching an insulation layer, and does not include a part laminating with the third conductive layer.