We prove that the flatness condition in Ocneanu's paragroup
theory for graphs with depth two is equivalent to existence of
the multiplicative unitaries in the theory of Baaj-Skandalis by
using ``Fourier transform'' introduced by A. Ocneanu. Moreover,
we realize a subfactor arising as a Kac algebra crossed product
from two Kac algebras dual to each other using the string algebra
construction.
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(C)2000 Nobuya Sato [e-mail:nobuya@mi.cias.osakafu-u.ac.jp]