This paper proposes CostFilter-AD, which introduces the concept of cost filtering, borrowed from classical matching tasks, to the unsupervised outlier detection (UAD) problem. We construct a matching cost volume between an input image and normal samples, and propose a cost volume filtering network that uses input observations as attention queries to suppress matching noise and capture subtle outliers. CostFilter-AD is designed as a plugin applicable to both reconstruction-based and embedding-based methods, and its performance is demonstrated on the MVTec-AD and VisA benchmarks.