Maximal operator in Musielak--Orlicz--Sobolev spaces

We study the Hardy-Littlewood maximal operator in the Musielak-Orlicz-Sobolev space $W^{1,\phi }({\mathbb{R}}^n)$. Under some natural assumptions on $\phi$ we show that the maximal function is bounded and continuous in $W^{1,\phi }({\mathbb{R}}^n)$.