@Ichiro: I checked your sample.obj file above and noticed that it contains face which consists of more than 5 vertices as follows:
[L:711]
f 62/217 10/218 24/219 26/220 25/221 6/222 27/223 30/224 63/225 64/226 65/227 66/228 67/229 31/230 54/231 58/232 60/233 59/234 52/235 68/236 69/237 70/238 71/239

The face of 3DGS MDL5 are all triangles, so if input OBJ contains face more than 3 vertice, OBJ->MDL converter like this software must split it into triangles. Currently this software can this operation only for faces which consists of 4 vertices.(Because it's mainly developed for OBJ exported from the Poser.) So I think it's the cause of the problem above.

Some 3D software's OBJ exporter has a option such as "Export all faces as triangle" or "Split face as triangle", and so on. So if your 3D software has such option, may be it's a closest solution of the problem above.

But anyway I think it's a important point for the future improvement of my OBJ2MDL itself...
So thank you for your report of the problem above.