To provide a compact actuator for driving a valve capable of driving opening of an adjusting valve (1) stably (2) with high precision and (3) with high response, being light in weight, and having high efficiency.

In this pneumatic and hydraulic actuator 10 for driving a valve for driving an output shaft 10A linearly by thrust of an air cylinder 11, hydraulic boosters 21A, 21B are provided on both sides of the air cylinder 11, each of both side piston rods 13A, 13B of the air cylinder 11 is connected with pistons 22A, 22B of each hydraulic booster 21A, 21B to increase oil pressure of each hydraulic booster 21A, 21B more than pneumatic pressure supplied to the air cylinder 11, a hydraulic cylinder 31 connected to both hydraulic boosters 21A, 21B is provided to connect the output shaft 10A to a piston rod 33 of the hydraulic cylinder 31 and supply oil pressure in each hydraulic booster 21A, 21B to each of both side oil chambers 34A, 34B nipping a piston 32 of the hydraulic cylinder 31.