Abstract: A method to discover the mesoscale system in a heavy rain event is suggested. That is to analyze the isochrone of the outbreak time of each rainy spell from the recording raingauge trace in the event. The detail in operation and the example of analysis are presented and compared with the analysis of hourly rainfall chart.It is indicated that the center in the hourly rainfall chart is mostly repres ented the microscale system such as convective cell so that it would be better to analyze the hourly rainfall center accompanied by mesoscale analysis. The isochone of outbreak time of rainy spell can expose the mesoscale system therefore it should be widely used as an effective means of mesoscale analysis.