PURPOSE: To introduce fluid into an ion source of a mass spectrographing device capable of electric control from outside without using a flowmeter, small, inexpensive, without time serial change and with heat resistance.

CONSTITUTION: This is a flow rate control valve to introduce fluid into an ion source of a normal closed mass spectrographing device which is provided with a supply hole A and a discharge hole B on a valve seat S separately from each other so as to open the valve seat S and elastically presses a valve body 2 made of an elastic plate from the back against this valve seat S by a pressing member 14, and a second pressing member 6 to press the valve body 2 from the side of the valve seat S against p pressing force of the pressing member 14 through the supply hole A is provided. A piezoelectric element 4 is arranged so as to control a position of this second pressing member 6 in the direction of the valve body 2 by the piezoelectric element 4.