The objective of this research was to compare the amount of extraneous water flowing into selected gravitational separate sewer systems which discharge domestic sewage from the settlement of single-family houses in Poland and France during a year with a standard annual amount of precipitation. The study included a total of four sewer systems, including two operating in Poland and discharging the sewage from settlements near Krakow in the Malopolska Province and two systems operating in France and discharging the sewage from settlements of single-family houses near Lorient, Brittany, Morbihan Department. As part of the analysis, an average daily tributaries of extraneous water into sewers under the research, values of addition and share of this water were compared. Analysis of the research results demonstrated that the hydraulic load of sewer systems with extraneous water does not depend on the age of the sewer system, but mainly on the number of house drains, and to a lesser extent its length. It was also determined that the average daily amount of extraneous water flowing into sanitary sewer systems in wet weather is 0,107 m3·d-1 per 1 sewer connection. The study showed that tributaries of extraneous water into the separate sewer system occurring during the intense precipitation are a major operating problem both in Poland and France.