Is there any way to heal ability drain without using magic (for example restoration)?
Nope.
Deus Ex Machina. Ability drain should be a rare thing in any Pathfinder game.
So ability drain doesn't heal naturally right?
Correct: you need magic to get rid of it, in no-or-low-magic campaigns DO NOT USE IT unless it's a part of a big over-arching plot.
Thank you all.
In low-magic worlds you might want to introduce some rare herbs or add feats allowing "surgical" restoration of ability drain if you really-really need to introduce ability draining challenges. Or you can downgrade ability drain to ability damage with slower healing - technically it shouldn't change CR much, as ability damage/drain rarely is healed during encounter and thus the total penalty to ability per encounter is what usually counts for CR purposes. Otherwise, there is no way of healing ability drain without magic.