The invention provides a mixing device allowing uniform mixing by using power material and liquid material through a kneader to produce resin composition with stable quality. The mixing device comprises a kneader (N) having a first supply opening (1) and a second supply opening (2) on the downstream of the first supply opening (1), a powder supply unit (F) connected with the first supply opening (1), a pressurized injection unit (P) connected with the second supply opening (2), a low temperature retaining unit (L) arranged between the first supply opening (1) and the second supply opening (2), a heating unit (H) arranged between the second supply opening (2) and the predetermined downstream portion, and a cooling unit (C) arranged on the downstream relative to the heating unit (H).