I want to know that why electric potential is not zero inside the surface as electric feild is zero.

A conductor has free electrons, and they experience a force only when the electric field is not zero. In a static situation, the free electrons gets itself distributed in such a way that, the electric field everywhere inside the conductor is zero.

In an electrostatic situation, wthin a conductor, both the electric field and volume density of charge vanish and just outside the surface, the electric field acts at right angles to the surface at every point. No work is done in moving a small test charge, within the conductor and on its surface. We find there is no potential difference between the two points inside or on the surface, which implies the potential being constant throughout.