An inkjet head driving method for an inkjet head, wherein the inkjet head includes a nozzle that jets ink and a pressure generator that changes a pressure on ink in a pressure chamber communicating with the nozzle in response to receiving a voltage signal having a predetermined unit drive waveform (Wn) and thereby causes the nozzle to jet an ink droplet, wherein the method comprises applying a voltage signal having a combined drive waveform, the combined drive waveform including multiple unit drive waveforms (Wn) each of which is the unit drive waveform, to the pressure generator and causing the nozzle to jet multiple ink droplets according to the voltage signal having the combined drive waveform such that the ink droplets unite and land on a recording medium as one droplet, wherein the unit drive waveform includes a first pulse waveform that causes the nozzle to jet an ink droplet and a second pulse waveform that pulls back the ink droplet jetted by the first pulse waveform in a direction opposite an ink jetting direction, wherein the first pulse waveform and the second pulse waveform each include an expansion part that expands the pressure chamber and a contraction part that is applied after the expansion part and that contracts the pressure chamber, wherein the combined drive waveform includes a first unit drive waveform being the unit drive waveform and a second unit drive waveform being the unit drive waveform applied after the first unit drive waveform, wherein a voltage amplitude (ΔV2) of the contraction part of the second pulse waveform in the second unit drive waveform is greater than a voltage amplitude (ΔV1) of the contraction part of the second pulse waveform in the first unit drive waveform, wherein an electric potential of the first unit drive waveform varies within a range not exceeding a predetermined reference electric potential, wherein the second pulse waveform of the second unit drive waveform has a part that is higher than the reference electric potential, wherein a last unit drive waveform in the combined drive waveform is the second unit drive waveform, wherein in the last second unit drive waveform in the combined drive waveform, the second pulse waveform has the part that is higher than the reference electric potential wherein in the last second unit drive waveform in the combined drive waveform, the second pulse waveform has a length of which is 1 AL, wherein an acoustic length is half an acoustic resonance cycle of a pressure wave in the pressure chamber. wherein the inkjet head includes a nozzle that jets ink and a pressure generator that changes a pressure on ink in a pressure chamber communicating with the nozzle in response to receiving a voltage signal having a predetermined unit drive waveform (Wn) and thereby causes the nozzle to jet an ink droplet, a nozzle that jets ink and a pressure generator that changes a pressure on ink in a pressure chamber communicating with the nozzle in response to receiving a voltage signal having a predetermined unit drive waveform (Wn) and thereby causes the nozzle to jet an ink droplet, wherein the method comprises applying a voltage signal having a combined drive waveform, the combined drive waveform including multiple unit drive waveforms (Wn) each of which is the unit drive waveform, to the pressure generator and causing the nozzle to jet multiple ink droplets according to the voltage signal having the combined drive waveform such that the ink droplets unite and land on a recording medium as one droplet, wherein the unit drive waveform includes a first pulse waveform that causes the nozzle to jet an ink droplet and a second pulse waveform that pulls back the ink droplet jetted by the first pulse waveform in a direction opposite an ink jetting direction, wherein the first pulse waveform and the second pulse waveform each include an expansion part that expands the pressure chamber and a contraction part that is applied after the expansion part and that contracts the pressure chamber, wherein the combined drive waveform includes a first unit drive waveform being the unit drive waveform and a second unit drive waveform being the unit drive waveform applied after the first unit drive waveform, wherein a voltage amplitude (ΔV2) of the contraction part of the second pulse waveform in the second unit drive waveform is greater than a voltage amplitude (ΔV1) of the contraction part of the second pulse waveform in the first unit drive waveform, wherein an electric potential of the first unit drive waveform varies within a range not exceeding a predetermined reference electric potential, wherein the second pulse waveform of the second unit drive waveform has a part that is higher than the reference electric potential, wherein a last unit drive waveform in the combined drive waveform is the second unit drive waveform, wherein in the last second unit drive waveform in the combined drive waveform, the second pulse waveform has the part that is higher than the reference electric potential wherein in the last second unit drive waveform in the combined drive waveform, the second pulse waveform has a length of which is 1 AL, wherein an acoustic length is half an acoustic resonance cycle of a pressure wave in the pressure chamber.