PURPOSE: To continuously and rapidly supply a predetermined amount of a fluid under predetermined pressure, by a method wherein one of left and right small diameter cylinder apparatus emits a fluid and the other one of them sucks the fluid when the large diameter piston of a large diameter hydraulic cylinder apparatus moves left and right.

CONSTITUTION: When the pressure oil sucked up by a pump P flows in the right end side of a large diameter hydraulic cylinder apparatus 2, a large diameter piston 4 moves to the left and, with this movement of said piston 4, left and right small diameter pistons 5a, 6a also move to the left under the same pressure. Hereupon, the inner volume of a right-hand small diameter cylinder apparatus 10 increases and not only paint is sucked in the apparatus 10 from a fluid suction line 13 but also the inner volume of a left-hand small diameter cylinder apparatus 9 decreases and the paint in the apparatus 9 is emitted to a fluid emitting line 16 to be supplied to an air spray gun A. When a change- over valve 22 is automatically changed over and the pressure oil flows in the left end side of the apparatus 2, the large diameter piston 4 moves to the right and, at this time, the paint is sucked in the left-hand small diameter cylinder apparatus 9.