  Let $\varphi: \mathbb R^n\times [0,\infty)\to[0,\infty)$ be such that
$\varphi(x,\cdot)$ is an Orlicz function and $\varphi(\cdot,t)$ is a
Muckenhoupt $A_\infty(\mathbb R^n)$ weight uniformly in $t$. In this article,
the authors introduce the Musielak-Orlicz Campanato space ${\mathcal
L}_{\varphi,q,s}({\mathbb R}^n)$ and, as an application, prove that some of
them is the dual space of the Musielak-Orlicz Hardy space $H^{\varphi}(\mathbb
R^n)$, which in the case when $q=1$ and $s=0$ was obtained by L. D. Ky [arXiv:
1105.0486]. The authors also establish a John-Nirenberg inequality for
functions in ${\mathcal L}_{\varphi,1,s}({\mathbb R}^n)$ and, as an
application, the authors also obtain several equivalent characterizations of
${\mathcal L}_{\varphi,q,s}({\mathbb R}^n)$, which, in return, further induce
the $\varphi$-Carleson measure characterization of ${\mathcal
L}_{\varphi,1,s}({\mathbb R}^n)$.
