To achieve a forecasting of water level of a river with a limited volume of calculation and at a high accuracy by numerical calculation.

A measuring apparatus 2 is so arranged to have a water measuring part 11 to measure water level of a river at a specified time interval and a rainfall measuring part to measure a rainfall at a specified time interval. A water level calculator 3 is so arranged to have a first accumulation part 31 in which a first expression is accumulated, a second accumulation part 32 in which a second expression is accumulated, arithmetic parts 33 and 35 which determine a passage resistance from the measured water level and the first expression and an inflow from the measured rainfall and the second expression to calculate the forecast water level of the river from the passage resistance and the inflow thus determined and updating parts 34 and 35 which compare the forecast water level calculated with the measured water to updata the first expression accumulated at the first accumulation part 31 and the second expression accumulated at the second accumulation part 32 based on the resulting errors.