PURPOSE: To improve heat exchanging efficiency with elongated residence time caused by low flow velocity and reduce piping work cost by a method wherein at the ends of a number of flow-down passages arranged in parallel along heat receiving plates with slightly downward grade in the sidewise direction, heat receiving medium liquid is received.

CONSTITUTION: Heat receiving plates are of long and slender shape sidewise. Flow-down water passages 2 are extended sideways with nearly straight line shape in the right and left directions from the middle location of the plates 1 and are sloped rightward and leftward respectively. Heat receiving medium liquid is caused to flow along this passages 2 towards the ends of the passages 2. The heat receiving medium liquid is supplied on the surface of the heat receiving plates by a liquid dispersion means 3. Thereby, a flow-down type heat collector which is long in sidewise direction and short in height can be used and the construction cost of piping work can be reduced when it is compared with a heat collector of which components are arranged laterally and which is used as before. Heat exchanging efficiency can be improved by elongated residence time caused by relatively slow velocity.