The possibility to excite high Q-factor resonances connected to high field enhancement factors in loss-free media, such as photonic crystal slabs, here is reported. In particular we demonstrate and experimentally prove the existence of this special type of resonances in the visible transmission spectrum of a very thin two-dimensional photonic crystal slab. For this purpose, we design and fabricate a square lattice array of holes in silicon nitride. Numerical simulations show that an extremely large field enhancement factor large as 700 times the amplitude of the incident wave, connected with high Q-factor resonances. The connection with the bound states in continuum phenomenon is highlighted.