I found a problem for using 'Skin' modifier. :(
Sometimes, I got a hole on the body, especially at the point of branch. Maybe this problem came from the complex structure at the point. I think the ways to solve it are,
1) Control the vertex distance and weight.
2) Separate the objects, body, legs, wings...
1) is not easy to control automatically, I think.
2) is easier, maybe.