In a method of stirring liquid in a droplet discharge head in which droplets of the liquid are discharged from a plurality of nozzle openings by driving piezoelectric elements provided to each of the nozzle openings, drive conditions of the piezoelectric elements are varied and the liquid that corresponds to the piezoelectric elements is caused to flow at different pressures. The strap row has a substrate contact structure that includes a substrate contact plug and a tap layer. The gate driver includes a gate shift register for generating scan pulses each having the first to third voltages, using the clock pulses, and supplying the generated scan pulses to the gate lines, respectively. The processing system also includes an EM wave receiving antenna configured to absorb the travelling EM wave after propagation through the process chamber. A photodetector device allows the control circuit to make this determination.