To provide a planetary mixer constituted so as to form a metal powder from a shaft seal part when treatment is performed by applying pressure to the inside of a tank or reducing the pressure in the tank.

A rotor (5) is attached to the main shaft (3) extending so as to pierce through a support cylinder (4). The rotor (5) has a gear case part (6) and a bearing case part (7). A blade shaft (8) is supported by a bearing case part (7) and a blade (28) is provided to the lower part of the blade shaft (8). An upper cylinder (12) is provided above the gear case part (6) so as to surround the support cylinder (4). A lower cylinder (13) is provided to the lower part of the bearing case part (7) so as to surround the blade shaft (8). Magnetic seal units (15) and (18) are provided between the upper cylinder (12) and the support cylinder (4) and between the lower cylinder (13) and the blade shaft (8), respectively.