This paper presents REWIRE, a novel method to solve the "data wall" problem, which is the difficulty of securing data for improving the performance of large-scale language models (LLMs). It generates synthetic data through guided rewrite by recycling low-quality web data discarded in the existing data filtering process to improve its quality. The DCLM benchmark experiments on 1B, 3B, and 7B scales show that the performance is improved by 1.0%, 1.3%, and 2.5% compared to using only filtered web data, and it is more effective than using twice as much web data. About 82% of the synthetic data is generated by converting low-quality documents that were previously discarded, and it outperforms other existing synthetic data generation methods (e.g., Wikipedia-style rewriting, question-answer synthesis, and knowledge extraction). This suggests that web text reuse is a simple and effective method for expanding LLM pre-training data.