PURPOSE: To improve accuracy of flow rate by preventing deflection of a valve stem, and also to improve vibration-proof property and durability by reducing bearing load for supporting the valve stem.

CONSTITUTION: The rotor 26 of a step motor 20 is constituted of a nut part 26b, a bearing 26c and a magnet 26a, and is supported rotation free on ball bearings 24, 25. A screw part 28a is formed on the left edge of a valve stem 28, and is screwed in a female screw of the nut part 26b. A valve body 32 is fixed to the right edge of the valve stem 28, the left edge of the valve stem is supported on the bearing 26c and its middle part is supported on another bearing 27, and the valve stem moves the valve body 32 by moving to right or left according to rotation of the rotor 26.