A BOD biosensor which uses irrunobilized yeasts for its receptor and a DO probe for its transducer has made it possible to measure BOD in wastewater in a short time and without any special analytical technique. Since being standardized as Japan Industrialized Standard (JIS) K 3602 in 1990. the BOD sensor has entered into a commercial stage. It has been reported that the BOD sensor is applicable to specific wastewater generated by the food industry, but it is unclear whether the BOD sensor is applicable to domestic wastewater, in particular. to secondary effluent from wastewater treatment plants. This paper discusses the applicability of the existing BOD sensor to domestic wastewater and the necessity of improving the existing BOD sensor. Based on the experimental evaluation of the actual samples in this paper, it was found that the BOD sensor requires modifications when it is applied to less biodegradable samples such as secondary effluents from wastewater treatment plants or river waters. Thus, several modifications were conducted. As a result, selections of microorganisms for the receptor and of BOD standard solutions for calibration resulted in improvement of applicability of the BOD sensor to less biodegradable samples. Using the renovated BOD sensor, satisfactory coincidence was observed between manually analyzed BOD5 and sensored BOD for secondary effluent and river water.