Transmission properties of the system of dielectric spheres periodically mounted in a metal slab are investigated. The electromagnetic wave can enter the dielectric spheres by tunneling effect and exists as a cavity mode. The surface plasmon polaritons can occur at certain frequency because of the periodically mounted dielectric spheres, which effectively modulate the metal surface to a periodical structure. When the frequencies of the cavity mode and that of the surface plasmon polaritons are close to each other, the coupling between them can greatly enhance both the electromagnetic modes, which results in a very strong electric field in the dielectric spheres of the upper layer. This strong electric field can further enter partially the dielectric spheres at the lower layer by tunneling effect, and can propagate out of the metal slab by the similar mechanism at the upper side of the metal slab, which causes additional increment in the transmission.