The present invention aims at providing a kneader capable of eliminating the cleaning process in the kneading process by preventing gushing of the filler through the clearance between the mixing tank and the pressurizing lid, stabilizing the blending ratio of the filler, and preventing defective dispersion due to fall and mixing of filler after the completion of kneading, for promoting improvement of dispersion and uniformity of kneaded materials such as rubber, plastics, etc. as well as stability of quality. In a kneader comprising a mixing tank 1 for storing the kneaded material, a pair of rotors 2, 2 disposed in parallel for kneading the kneaded material stored in the mixing tank 1, and a pressurizing lid 3 inserted slidably in the opening at the top of the mixing tank 1, the sliding portion of the pressurizing lid 3 against the mixing tank 1 is divided into a plurality of stages (upper, lower), packings P1, P2 are disposed for reducing the clearance between the respective pressurizing lids and the side wall of the mixing tank as much as possible, and an air reservoir 33 is formed between said upper and lower lids 31, 32.