Always show the inner of a model

Hi all, I made a model with Maya and import it into Unity, but I can only see the inner face of the model, the surface that faced to camera disappeared, what’s wrong with my model?

I have found the cause of the problem. The normals direction of my model opposite.

I fixed it in Maya by the following steps:

1.Choose Polygon mode.
2.Select your model.
3.Click the Normal meun.
4.Click Revers sub menu.

Save your change, the problem resolved.