  We investigate an N=4 U(N)_k x U(N+M)_{-k} Chern-Simons theory coupled to one
bifundamental hypermultiplet by employing its partition function, which is
given by 2N+M dimensional integration via localization. Surprisingly, by
performing the integration explicitly we find that the partition function
completely factorizes into that of the pure Chern-Simons theory for two gauge
groups and an analogous contribution for the bifundamental hypermultiplet.
Using the factorized form of the partition function we argue the level/rank
duality, which is also expected from the Hanany-Witten transition in the type
IIB brane realization. We also present the all order 't Hooft expansion of the
partition function and comment on the connection to the higher-spin theory.
