A kneader according to the present invention has a kneader main body that has a casing having a kneading chamber and a kneading portion that is disposed in the casing and kneads a material in the kneading chamber. The material supplied to the kneading chamber includes a resin material, an inorganic additive added to the resin material, and a coupling agent for enhancing affinity of the inorganic additive for the resin material. The kneader further has a kneading monitoring portion that monitors a kneading state of the material by detecting a reaction product that is generated by a reaction between the coupling agent and the inorganic additive.