The time delay properties of a Fabry-Perot interferometer are investigated. We found that the group velocity of light through a Fabry-Perot interferometer can be reduced to 10(-4) of the light speed in vacuum and the time delay is 210 ns, when the reflectivity is 0.999 and the distance between two mirrors is 1 cm. The system is analogous to the recently proposed one-dimensional photonic band-gap structures with a defect [Zhu et al. Opt. Commun. 174(2000)139].