Provided is a method for developing a secondary organ by using a non-human animal in which organ formation is inhibited, for the purpose of establishing a process for producing a functional cell such as a β cell within the body of an animal such as a pig, the method including the step of raising a newborn or a fetus of the non-human animal in which organ formation is inhibited by complementing at least a part of the function of the organ whose formation is inhibited.
