A detector for detecting sodium hypochlorite concentration includes a first electrode, a second electrode and a processing unit. The first electrode and the second electrode are disposed opposite each other, and are soaked in a sodium hypochlorite solution under detection. An electrical path is formed among the first electrode, the sodium hypochlorite solution and the second electrode. The processing unit, electrically connected to the first electrode and the second electrode, measures a plurality of ions in the sodium hypochlorite solution transmitted in the electrical path to obtain a conductivity of the sodium hypochlorite solution and to accordingly calculate concentration of the sodium hypochlorite solution. Compared to a conventional measurement apparatus adopting an optical detector, the invention not only can be fabricated at low production costs, but also can instantly obtain the detected concentration without involving analysis of an externally connected computer.