An ecosystem, especially a food web, is essentially characterized as a many-body system in which the members interact with each other under the limitations of the energy and resources. We introduce a coevolutional population dynamics model for food webs which contains energy-conserving interactions, energy dissipation, and rules for changing the degrees of freedom (extinction and mutation). In this model, the diversity of the system increases spontaneously. The statistical properties of the system, such as the distribution of the life time of the species, are also discussed.