A method of measuring the number of bacteria according to the present invention includes step (a) through step (d). First, in step (a), a to-be-measured sample (specimen) including a predetermined bacterial strain (such as Escherichia coli or coliform bacteria) is added to a predetermined medium (such as a medium used for a specific enzyme substrate culture medium method). In step (b), a current value flowing through the medium added with the sample is measured with an oxygen electrode at a predetermined temperature and at a predetermined constant voltage. In step (c), time required is measured for the current value that has decreased temporarily after starting the measurement of step (b) to increase thereafter to exceed a predetermined threshold value. In step (d), the initial number of bacteria of the bacterial strain included in the sample is calculated based on the time required.