An ink jet system printer of the charge amplitude controlling type wherein a charge amplitude on ink drops charged by the application of phase detection signals to a charging electrode is sensed and then the relationship in phase between charging signals and ink drop separation is compensated in a manner to ensure that the ink drops are charged at an optimum state at all times. In the ink jet system there is provided a phase synchronization detection circuit for producing phase synchronization signals only when the phase relation is maintained at an optimum state due to the compensation, and a phase synchronization time period measuring circuit for determining whether the phase synchronization signals occur in sequence during a specified period of time and for producing printing ready signals if an affirmative answer is obtained.