Thank you for the answer.
The sample, the polygon was tried on one.
But I would like to know which polygon is in front,
when two or more polygons exist.
Then, I watched the "buff".
It means distance of the depth direction on 3D-space.
HD graphics 4000 returned zero,
however other video cards returned non-zero.
In the case of zero value,
It won't know the positional relation on 3D-space of two or more polygons.