To perform efficient assembly by inserting a plurality of straightening devices into a tube for mounting by a single insertion operation, and to cope with the miniaturization of the straightening devices and a change in the specifications of the number of the straightening devices to be arranged.

A straightening device unit has an annular recess 19 for accepting the straightening device for straightening fluid flowing in a channel 13 of a flowmeter 11 on one end face, an annular projection 20 that projects having a size fitted to the annular recess 19 on the other surface, and a spacer 17 having a fixing means that is engaged to the tube of the channel 13 for fixation. The straightening device unit is tentatively assembled to the straightening device unit U2, where the spacer 17 and the straightening device are alternately connected in the axial direction and are integrated cylindrically.