The temperature in the reaction vessels of the automatic analyzer using either one or two compartment reaction systems was monitored using a pH indicator. The heat conduction of vessels and thermostat system disturbs the increase of aqua temperature in reaction vessels. This is an effective method to monitor the stability of aqua temperature in the reaction vessels. In addition, an automatic analyzer may be needed to change the starting-point of the reaction measuring the absorbance in the enzyme assay system. In particular, when analytical factors of the measurement of enzymes are placed into the instrument, the aqua temperature in the reaction vessels should be confirmed to return to a basal level after adding reagents and mixing the solution.