The present invention addresses the problem of providing an eggshell membrane solubilization method that is capable of solving the problems associated with carrying out treatment using acids and alkalis, or problems associated with the processing methods of the conventional art that use proteases; in other words, an eggshell membrane solubilization method that is capable of solving at least one of the following problems: (1) the need for pretreatment such as pulverization, sonication or boiling; (2) the need for prolonged treatment; and (3) a low decomposition rate (approximately 20%). Eggshell membranes are efficiently solubilized by using a protease in combination with a reducing agent.