A COD conversion equation holding unit in a computing unit of a COD measuring device holds a COD conversion equation that indicates a relationship between total-absorbance that is linearly combined by multiplying an absorbance value at each of a plurality of wavelengths by a weighting factor and a COD value, and a conversion unit finds a converted COD value by using the COD conversion equation, and outputs the resulting value. Preferably, a COD conversion equation calculation unit calculates a COD conversion equation based upon the relationship between the total absorbent and an actual measured COD value in sample water, and a correlation calculation unit finds a correlation with respect to each of a plurality of COD conversion equations calculated by the COD conversion equation calculation unit. In this case, the COD conversion equation holding unit holds the conversion equation having the best correlation among the COD conversion equations calculated by the COD conversion equation calculation unit as a COD conversion equation for the sample water.