Let X be a ball quasi-Banach function space on $${{\mathbb {R}}}^n$$ and assume that the Hardy–Littlewood maximal operator satisfies Fefferman–Stein vector-valued inequality X, let $$q\in [1,\infty )$$ $$d\in (0,\infty . In this article, authors prove that, for any $$f\in {\mathcal {L}}_{X,q,0,d}({\mathbb {R}}^n)$$ (the Campanato-type associated with X), Littlewood–Paley g-function g(f) is eith...