Thanks Shiko.
I have a shapefile with more than 30000 polygons, so I'm trying to find a way to do it automatically and not manually.
I could do it manually but I have to repeat the process several times (I have 100 different shapefiles), so, it could be great if I find a way to solve that.
The dissolve field is optional, and if you don't specify any, then it dissolves the adjacent polygons into bigger ones (just whay I want BUT it doesn't consider as a neighbour the polygons that only share a point with the other ones).
Any additional idea ?
Thank you.