PURPOSE: To measure reacting weight with high accuracy even if the time when temps. are low continuing for a long period of time by inhibiting the operation of reacting weight by a microcomputer if the reacting temps. are lower than set lower limit values.

CONSTITUTION: The temps. detected at required points in the inside of reacting systems, the surfaces of the reacting system, vessels in the reacting systems, etc. by temp. detectors 1W6 are applied via a multiplexer 7, an A/D converter 9, etc. to a CPU10. The CPU10 counts reacting weight according to these temps. The information on the lower limit temp. set with a lower limit temp. setter 27 is also applied to the CPU10 and when the input detected temps. drop lower than the set lower limit temp., the counting of the reacting weight is inhibitied and a reacting weight 0 is outputted. Thus, even if the time when the temps. are low continue for a long period of time, the reacting weight is measured with high accuracy without accumulation of errors in the calculated value in accordance with approximate reaction formulas.