It is an object of the present invention to provide a process for decomposing vegetable waste materials easily, which solves problems such as the problem of treatment costs, the problem of energy, an environmental problem and the problem of time for treatment which are difficult to be solved by prior art.The present invention provides an agent for decomposing plant fibrous materials, which contains plant fibrous materials decomposing enzymes derived from a microorganism, but not the microorganism. In addition, the present invention provides a process for treating vegetable waste materials comprising allowing the agent for decomposing plant fibrous materials to act on vegetable waste materials.