As is well known, the wave function of the Universe dictated by the Wheeler-DeWitt equation has a difficulty in its probabilistic interpretation. In order to overcome this difficulty, we explore a theoretical possibility of the second quantization of the Universe, following the same passage historically taken for the Klein-Gordon particles and the Nambu-Goto strings. It turns out that multiple production of universes is an inevitable consequence even if the initial state is nothing. The problematical interpretation of the wave function of the Universe is circumvented by introducing an internal comoving model detector, which is an analogue of the DeWitt-Unruh detector in quantum field theory in curved space-time.