A printing element substrate, comprising a plurality of units configured to print on a printing medium based on print data, each of the plurality of units, including a printing element configured to print on the printing medium, a first transistor configured to operate as a source follower upon receiving a voltage at a gate terminal of the first transistor, and supply a current to the printing element, and a second transistor configured to control supply of the current to the printing element in response to a control signal input to a gate terminal of the second transistor.