Weird mirror line when subdividing?

Hello. I am making this crappy spider-man model but i encountered a problem when i subdivided, after applying the mirror modifier to the model. When i subdivide, a line appears between the model. How do i fix that?
Oh, and any suggestions on how i would model the head?
Any help would be greatly apreciated.
Blender ver: 2.62

That line appears to be an indentation of the center line, in which case it would be a matter of moving the vertices out to eliminate the indentation. If that is not the case, then another possibility is that your center edge loop was not truly on the center line and/or the cursor was not truly on the center line before you mirrored.

To perfectly center your center edge loop before mirroring, follow this recipe:

In Edit Mode:
  1. select the center edge loop and use scale (x) to flatten it
  2. shift S - cursor to selected (places cursor on edge loop plane)
    In Object Mode:
  3. Object Tools / Origin - Origin to 3D cursor
  4. shift S - cursor to center
  5. shift S - selection to cursor
  6. Add Modifier / Mirror

If none of that works, you might also consider attaching the .blend file so others can check it.

Thank you for your reply John Howard.I did everything you said but, unfortunately it didn’t work.
I’ll post the .blend file so the problem can be examined more thoroughly.

The problem is that you have internal faces :

Delete them and the problem will go away

Problem solved! Thank you very much, for your quick reply Sanctuary. I did what you said, i deleted the internal faces and the line is now gone!
Once again, thank you very much!