In a fluid container comprising a container main body, a first piston member which defines a fluid containing chamber of a variable internal volume in cooperation with associated walls of the container main body, and a content expelling tube for conducting the content of the fluid container to a content outlet, a second piston member is placed inside the content expelling tube so that the second piston member may be moved inside the content expelling tube to take out the content from the content outlet when the first piston member has reached its stroke end and the content inside the fluid containing chamber has been all taken out. Thus, the content expelling tube may be conveniently employed for guiding the movement of the first piston member without reducing the inner capacity of the fluid container. If the movement of the second piston member is detected with an appropriate sensor, it is possible to detect the depletion and/or the imminence of the depletion of the content of the fluid container in a reliable manner because the movement of the second piston member is typically more pronounced than the first piston member which normally has a substantially larger diameter than the second piston member.