To efficiently print by surely preventing a thermal breakdown of a driver circuit of an ink jet printer and shortening a time required for a cooling operation.

T1-T4 indicate temperatures of drivers for driving an ink jet head, Lim1 indicates a transition threshold 1 signal (driver breakdown precaution temperature) outputted from a transition threshold setting part 46, and Lim2 indicates a transition threshold 2 signal (driver breakdown caution temperature). When the temperature T1-T4 of the driver is measured and sent to a CPU 45, a comparison operation of the Lim2 and T1-T4 is carried out. When T1-T4≥Lim2 is held, the printer is instantly shifted to the cooling operation S1 using an air-cooling mechanism 38 or a water-cooling mechanism. When the operated result becomes T1-T4<Lim2, printing is continued. The temperature T1-T4 is compared with the Lim1 after printing is finished. When T1-T4≥Lim1 is held, it is instantly shifted to the cooling operation S2, ink is replenished in a sub tank and, the printer is naturally cooled.