PURPOSE: To provide a unit which is suitable for intermittent backwash depending on small quantity of filtrate in especially small-scaled filtration and intermittent reflux depending on small quantity of distillate in small-scaled rectification.

CONSTITUTION: Liquid flowing from an ordinary flow-in port 7 to an ordinary flow-out port 6 through a reservoir 8 pushes up the first diaphragm 5 when liquid being higher in pressure than the ordinary flow-in port 7 flows in from a back flow liquid feed port 9, and a valve section 1 is thereby pressed against the first valve seat 10 through the first spring 4 to stop a flow from the ordinary flow-in port 7 to the ordinary flow-out port 6. Next the liquid is fed from the back flow liquid feed port 9 to make the first diaphragm 5 flow back the liquid in the reservoir 8 toward the ordinary flow-in port 7, and the back flow is accomplished within a range not exceeding the volume of the reservoir 8 so long as the liquid from the back flow liquid feed port 9 actuates the first diaphragm 5.