PURPOSE: To compensate flow rate fluctuation of high frequency by forming a port on a side surface of a cylinder, and providing an extention/contraction member being in contact with a thin wall at its one end and contact with a wall on an outer wall side of the cylinder at the other end, in the port, because of significantly speedy response of the extention/contraction member compared to that of a motor.

CONSTITUTION: A port 21 having a tapered female screw is formed on a side surface of a cylinder 3. A male screw engageable with the female screw of the port 21 is formed on one end of a cylindrical connector 22. A recession having a stepped portion 23 is formed inside the connector 22, and a thin wall made of thin metal is formed alignedly on the stepped portion 23. An extention/contraction member 25 is such as piezoelectric element is provided being in contact with the thin wall, and fixed to the inside of the recession by means of a cap 26 connected to a connection portion such as a screw on an end part of the connector 22. A displacement sensor 29 is provided in a space 28 for measuring a displacement rate of the thin wall. Mass of the thin wall is significantly small compared to that of a plunger 7, while the extention/ contraction member 25 such as piezoelectric element speedily responds compared to the response of a motor 10.