The manufacturing method includes a granulation step. The granulation step is a step of forming a plurality of granules by performing extrusion granulation on a granulation material using a granulation machine. The granulation machine includes a die and a roller. A through hole that allows the granulation material to pass therethrough is formed in the die. The roller pushes the granulation material into the through hole while rolling on the surface of the die. In the granulation step, low-speed pushing in which the granulation material is pushed while the roller rolls at a first speed, and high-speed pushing in which the granulation material is pushed while the roller rolls at a second speed that is higher than the first speed are executed.
