Abstract: Provided is an actuator including a ball-type thrust amplifier therein that controls a valve in which a high-pressure fluid flows, and having less wear between members and a long lifespan. This actuator for a valve includes a thrust amplification mechanism therein, in which the thrust amplification mechanism includes a disk, a ball presser, a stem, and a plurality of balls that are sandwiched by and in contact with an upper surface of the disk, a tapered surface of the ball presser, and a tapered surface of the stem and disposed to move outward as the stem moves downward, the surfaces of the parts constituting the thrust amplification mechanism are coated with grease containing an additive, and the additive of the grease includes at least chloroalkane.