To solve such a problem that a person, who is not an expert, can not easily understand the conventional river information system since the display information is indicated with only the numerical values of raw data, tables, and graphs.

The explanation displaying system is provided with an observation means 1 placed at a prescribed location, a river data analyzing means 2 analyzing data such as an amount of rainfall and water level measured with the observation means 1, a river information communicating means 5 automatically distributing river information 3 obtained from the river data analyzing means 2 to a prescribed platform 4, and an information explanation displaying means 6 fully automatically displaying in real time message supporting information 7 such as comment and a caution alerting sentence corresponding to the river information 3 momently changing on the screen of the platform 4. The information explanation displaying means 6 displays the message supporting information referred to the teachings of an accident in the past, a levee protection alarm, and a flood forecast sentence on the screen of the platform 4.