  We study Yang-Baxter deformations of the Nappi-Witten model with a
prescription invented by Delduc, Magro and Vicedo. The deformations are
specified by skew-symmetric classical $r$-matrices satisfying (modified)
classical Yang-Baxter equations. We show that the sigma-model metric is
invariant under arbitrary deformations (while the coefficient of $B$-field is
changed) by utilizing the most general classical $r$-matrix. Furthermore, the
coefficient of $B$-field is determined to be the original value from the
requirement that the one-loop $\beta$-function should vanish. After all, the
Nappi-Witten model is the unique conformal theory within the class of the
Yang-Baxter deformations preserving the conformal invariance.
