A plunger arrival target time adjustment method for use in conjunction with a gas-producing well includes the steps of setting times of A valve open and close states, setting times of B valve open and close states where the time of B valve open state occurs separately from and in succession after the time of A valve open state, setting a target time for arrival of a plunger starting with opening of the well upon converting the A valve to the open state and ending with sensing of arrival of the plunger at an upper terminal position of the well, measuring travel time of the plunger from the opening of the well to the sensing of plunger arrival irrespective of whether the arrival occurs during the time of A valve open state or the time of B valve open state, and setting a new target time for plunger arrival based on a predetermined relationship of the measured plunger arrival travel time to the previously set plunger arrival target time.