PURPOSE: To perform successively a monitoring and controlling by forecasting the measuring value of the present time from the ratio of measuring value of the time immediately before and the measuring value before 24hr and the measuring values of the present time and the same time of the previous day in case of performing a maintenance during operation.

CONSTITUTION: A physical quantity to be measured is measured with a primary transducer 1 and converted into an electrical signal with a secondary transducer 2 and the measuring value is transmitted successively together with time to a measurement forecasting means 4. The measurement value forecasting means 4 judges either it is under regular operation or on maintenance and in case of under maintenance the measuring value of the present time is forecasted and displayed on a displaying device 3 from the ratio of the measuring value of the time immediately before and that 24hr before and the measuring values of the present time and the same time of the previous day. The continuation of the controlling and monitoring of the quantity to be measured is thus enabled even under maintenance condition and the suspension of operation is avoided. It is suitable for a plant of a power station of 24hr cycling, etc.