Abstract: An abnormal combustion detecting construction for a burner is disclosed which is capable of directly detecting a variation of an oxygen concentration in a burner to positively and effectively sense abnormal combustion, particularly, that causing an excessively or abnormally increased combustion flame. The abnormal combustion detecting construction includes an oxygen sensor having one electrode arranged at a height in a combustion chamber of a burner which causes only the excessively or abnormally increased flame to reach the one electrode and the other electrode arranged above a partition plate to be constantly isolated from any combustion flame in the burner, so that a variation of an oxygen concentration in the combustion chamber may be directly detected.