A multilayer flow path member of an ultrasonic fluid measurement apparatus and an ultrasonic fluid measurement apparatus capable of enhancing the measurement accuracy of the mean flow velocity are provided. When a multilayer flow path member 30 placed in a measurement flow path 14a shaped in a rectangular cross-section pipe of an ultrasonic fluid measurement apparatus 10 is partitioned into a plurality of flat flow paths 14e by partition plates 32 attached to a frame 31 along a flowing direction, the partition plates 32 are provided so as to face inner faces 15f and 17a of the measurement flow path 14a. Thus, the partition plates 32 are exposed and face the inner faces 15f and 17a of the measurement flow path 14a and thus the space between the exposed partition plate 32 and the inner face 15f, 17a of the measurement flow path 14a becomes the highest-stage or lowest-stage flat flow path 14e. Therefore, a problem of degrading the measurement accuracy as a fluid flows into the space between the frame 31 forming a part of the multilayer flow path member 30 and the inner face 15f, 17a of the measurement flow path 14a as before does not occur and accordingly the measurement accuracy can be enhanced.