A case study is made for heavy rain which occurred in the area around Owase in the Kii Peninsula from the 9th to the 11th of September, 1971, using data from autographic raingauges with the aid of PPI radar photographs and the time variations of drop-size distribution and electric field intensity measured at Owase. Results of analysis show that travelling cumulonimbi produced heavy rain in Owase area, as if rainfall from it were amplified there, on account of the interaction with surrounding precipitating clouds of low top which formed only around Owase and in which the contribution of raindrops of 1 to 2mm in diameter to rainfall intensity was very large. It can be inferred that very efficient mechanism of rainfall formation was realized in Owase area as a result of the modification of cumulonimbus due to orographic effect. Mechanism of orographic enhancement of rainfall is discussed. Further it is pointed out that during the period of heaviest rain electric field intensity has the time variation of long time scale which does not reflect the passage of individual cumulonimbus travelling from the south with the interval of two to four hours successively.