some artefacts with transparency

i draw a box composed by 3 quads at each face (like three cubes stacked ).
i want to make it translucide.
i use alpha blending, but it don’t works correctly.

my object is translucide, but within the box, quads separating each pseudo-cube are visibles (i never defined those cubes ) .
why do i have those artefacts ?
how can i supress them ?

you sould post your source, without it, it is hard to find the bug