A nozzle blockage preventing unit is disposed at one end of the travelling zone of a carriage so as to confront a nozzle unit mounted on the carriage when the carriage is held stationary at the end of the travelling zone. The nozzle blockage preventing unit includes a cam mechanism and a cam follower plate which moves toward the nozzle unit when the cam mechanism rotates. A capping rubber member is secured to the cam follower plate such that the capping rubber plate contacts the nozzle unit when the cam mechanism is rotated, thereby forming a closed chamber covering an orifice formed in the nozzle unit. A protection liquid is filled in the closed chamber in order to prevent the drying of the ink liquid contained in the nozzle unit when the ink jet system printer is placed in a non-operating condition for a considerably long period of time.