To provide a method for counting vial cells to enable accurate monitoring of microorganisms in a process for the production of valuable substances or the waste water treatment using microorganisms, especially meso- thermophile or thermophile bacteria.

The counting of viable cells in a specimen is carried out at a specimen temperature Tc (°C) between 40°C and 85°C by a plate count method while keeping the temperature Ts (°C) of the medium for plate count to Tc-10°C<Ts<Tc+5°C during the measurement.