In recent years, many studies have been conducted on the effects of additives on the anaerobic digestion process. In this study, steelmaking slag was used as an additive in the anaerobic digestion of sewage sludge and its effect was investigated. Batch anaerobic digestion experiments were conducted by adding converter slag and blast furnace slag with different compositions. The results showed that the addition of converter slag increased the CH₄ content in the biogas by removing CO₂ from the biogas through the carbonation reaction between the Ca contained in the slag and the CO₂ in the biogas. When converter slag (2 g/L) was added to mixed sludge anaerobic digestion, the CH₄ content increased from 66% to 71% and the CH₄ yield increased 1.8 times compared to the control. On the other hand, the addition of blast furnace slag had no effect. The Fe content of the converter slag was higher than that of the blast furnace slag, suggesting that Fe compounds in the converter slag may have contributed to the increase in CH₄ yield.