This disclosure provides a method for producing a cross-linked rubber for the additively fabricated object that can be easily produced with good dimensional accuracy and simplicity, even using a general-purpose uncross-linked rubber composition. This producing method is a method for producing a cross-linked rubber, in which an additively fabricated object of uncross-linked rubber is heated in a liquid to produce an additively fabricated object of cross-linked rubber, wherein the method includes, heating the additively fabricated object of uncross-linked rubber within the range not exceeding the saturated vapor pressure by controlling the temperature and pressure of the liquid, obtaining the equivalent cross-linking amounts at regular intervals from the start of heating using the following Formula (1), accumulating them and calculating the cumulative cross-linking degree, and stopping the cross-linking reaction when the cross-linking degree of the additively fabricated object of cross-linked rubber reaches the required cross-linking degree. U = ∑ i = 1 n e − E R 1 T − 1 T 0 . t i t 0