  We investigate the correspondence between superstring theory on pp-wave
background with NSNS-flux and superconformal field theory on a symmetric
orbifold. This correspondence can be regarded as the ``Penrose limit'' of
AdS_3/CFT_2 correspondence. Superstring theory on the Penrose limit of AdS_3 x
S^3 (x M^4) (M^4 = T^4 or K3) with NSNS-flux can be described by a
generalization of Nappi-Witten model. We quantize this system in the covariant
gauge and obtain the spectrum of superstring theory. In the dual CFT point of
view, the Penrose limit means concentrating on the subsector of almost BPS
states with large R-charges. We show that stringy states can be embedded in the
single-particle Hilbert space of symmetric orbifold theory.
