PURPOSE: To provide a rubber kneader equipped with a recovery tank and a recovery circuit of the pressurized air in an air cylinder by a system wherein the recovery tank and the recovery circuit to recover and reuse the pressurized air in the air cylin der are equipped.

CONSTITUTION: Firstly, a piston 10 or a floating weight 8 is raised by introducing pressure air, which has been supplied to a recovery tank 24, into an air chamber 13 through a selector valve 23 in the state that both the coil 20A of the selector valve 20 and the selector valve 23 are turned ON and by exhausting the air in an air cham ber 12 through a silencer 18. Secondly, after rubber and the like are charged, the floating weight 8 lowers so to reach the rubber by supplying the pressure air in the tank 24 to the air chamber 12 by turning OFF the selector valves 20 and 23 and ON selector valves 21 and 22 and by exhausting the pressure air in the air chamber 13 through the silencer 18. Thirdly, high-pressure air is fed from pressure air source to the air chamber 12 by turning OFF the selector valve 22 and ON the coil 20B of the selector valve 20 so as to pressurize the rubber in order to press the rubber in a chamber 2 to knead the rubber. After the finish of kneading, the pressure air in the air chamber 12 is recovered by turning OFF the selector valves 20 and 21 and ON the selector valves 22 and 23.