A substrate for an ink jet recording head, the substrate provided with an electrothermal transducer for generating thermal energy used for discharging ink and a driving circuit for driving the electrothermal transducer, the substrate having a logical circuit for outputting, at a second voltage amplitude level based on an input signal of a first voltage amplitude level, a selection signal for designating the electrothermal transducer to be driven, a driving circuit for driving the electrothermal transducer based on the selection signal from the logical circuit in units of blocks, and a buffer circuit which has current supply capability enabling gates fed with one selection signal to be all driven in the driving circuit and relays the selection signal between the logical circuit and the driving circuit.