Where did you buy this game from? eBay? Are you sure it's a real disc and not a copy?
If this is a real disc then Bigmack's right, depending on the model of your PS2 the lens is probably fucked. You can get replacement lens but they're quite expensive and taking a PS2 apart is easy, getting it back together with no parts leftover is another story.
One thing I would try if I were you is start your PS2 with no disc in the drive, press TRIANGLE, highlight the "Playstation Driver" option, press TRIANGLE and change the disc speed, if it's fast make it slow, if it's slow make it fast. It's about the only thing you can really change without pulling your PS2 apart.
Failing that if you dont want to fix it yourself, take it to a shop to get it fixed.
Peace