To easily and quickly change the substantial fluid passage sectional area of an internal passage without changing the design of a tubular body itself, when the substantial fluid passage sectional area of the internal passage of the tubular body is changed.

This structure is provided with a partition member Q1 mounted on the internal passage A of the tubular body P1 approximately along its longitudinal direction for partitioning the internal passage of the tubular body into a fluid passage part As where the fluid passes and a fluid non-passage part An where the fluid does not pass substantially, a driving device 10 mounted outside of the tubular body and connected to the partition member for driving the partition member, and a seal member Rb for sealing a connecting part of the driving device and the partition member, and the driving device drives the partition member to be inclined to the longitudinal direction of the tubular body.