A thermal recording medium comprising a as a developer a sulfonyl compound represented by general formula (1) or a polyvalent metal salt thereof. ##STR1## where X represents H or a lower alkyl group, R.sub.1, R.sub.2 and R.sub.3 represent H, halogen atom, a hydroxyl group, a lower alkyl group or a cycloalkyl group, R.sub.4, R.sub.5 and R.sub.6 represent H, halogen atom, a lower alkyl group or a cycloalkyl group, and R.sub.7 represents H or ##STR2## where R.sub.4, R.sub.5 and R.sub.6 are as defined above. The thermal recording medium has a high sensitivity and a lower background blush during storage, and also has excellent stability against heating, moisture and plasticizer.