Wrongly sized crystal. A tension ring won't keep it in, afaik.
crystal by itself completely fit to the case. actually I have pull the crystal to remove. Provided that, should I buy new crystal and tension ring?
Unless they don't have Omega parts access, then they will probably use a generic crystal which will likely fall off again.
If it is an old crystal, I believe it could be down to shrinkage. Or it could be a veeeeery slightly too small diameter crystal.
EDIT: Is it an Omega crystal? If it is a front-loading case, I would be very careful in finding the correct type and size.
Doesn't look like a high valued watch, some glue will help save lots of money and headaches.