Check valve

A check valve comprising;

• a filtering second plastic film, a third plastic film and a fourth plastic film, said third and fourth plastic films being shorter than said first plastic film respectively and arranged in this order on one surface of said first plastic film;
• the lower edges and the right and left side edges of said first and second plastic films being joined together to define a discharge path therebetween and to define a valve seat portion in the upper portion of said first plastic film;
• the lower edges of said third and fourth plastic films being joined to said second plastic film at its portion between its upper and lower edges and the right and left side edges of said third and fourth plastic films and being joined to the right and left side edges of said second plastic film to define an upper opening between said first and fourth plastic films, to define a check flap in the portion of said third plastic film extending upwardly beyond the upper edge of said second plastic film for cooperation with said valve seat portion of said first plastic film, and to define a pocket between said third and fourth plastic films for pressing said checking flap against said valve seat portion, and
• the portion of said second plastic film in the side lower than the lower edges of said third and fourth plastic films being exposed for functioning as a filter surface.