PURPOSE: Simplification of construction by using piping provided with a detour and a by-path at an intermediate point.

CONSTITUTION: At an intermadiate point of a piping 1, there are provided U-type detour 2 and a by-path tube 3. The length or capacity of the U-type detour 2 is half that of water present independently. Orifices 4, 5 are provided at the inlet of the U-type detour 2 and at the outlet of the by-path tube 3 so that the flow rate of the U-type detour 2 is equal to that of the by-path tube 3. When water present independently in the piping 1 in the length L is reached the inlet of the orifices 4, 5, at the outlet 7 of the by-path tube 3 the effluent water from the orifice 5 of the by-path tube 3 is mixed with an oil 8 originally present in the U-type tube 2 in equal quantities of water and oil and the mixture is extruded. Thus, the water is diluted to twice the volume of water originally present and is extruded.