PURPOSE: To discharge an ink drop always stably and to prevent ink at a tip of a nozzle from drying and solidifying in capping at non-printing time by a method wherein an air pump is provided in which an air flow inlet on a head housing is connected to an exhaust side via a three port valve and an air flow outlet on a cap is connected to a suction inlet.

CONSTITUTION: A cap 16 covers an ink drop discharge opening 19 on a head housing 24 by operation of a self holding type solenoid 2 at a head stand-by position, and a small chamber is formed by a shielding rubber 18. At the same time, an outlet port 9 of a three port valve 11 is closed and an open air port 7 is opened. Then, a pump 1 is electrified and suction is started from an air flow outlet 21. Further, an air flow flowing via an exhaust tube 23 flows out by an open air open port 7 of the three port valve 11. Thereby pressure of an air chamber becomes negative, and ink is sucked out from a tip nozzle of a head 20. Thickened ink and a bubble are discharged from an inside of the nozzle. The discharged ink drop onto an absorbing material 15, and relative humidity in the small chamber in capping is raised. Thereby, a damping effect is shown to an ink jet head 20, and drying, solidifying, and thickening of the ink can be decreased.