To provide a method and a device for sorting/separating mixed plastic including different kinds of plastic materials with a specific gravity close to each other.

The mixed plastics 3 obtained by grinding/mixing different kinds of plastics with a specific gravity close to each other are supplied by a belt conveyor 4 and a high-frequency electric power is applied to a pair of electrodes 2a and 2b arranged allowing passage of the upper part of the belt conveyor 4 in between by means of an electromagnetic wave oscillation circuit 1. The mixed plastics 3 passing through the gap between the electrodes 2a and 2b are subjected to dielectric heating and the plastics 8 showing a high dissipation factor are melted by dielectric heating, while the other plastics with a low dissipation factor does not melt. The plastics with a low dissipation factor are recovered into a specific gravity separation water trough 13 by natural drop or air blowing 12. After that, the plastics 8 with a high dissipation factor are released from the belt conveyor 4 by means of a scraper 5 and recovered by a recover vessel 7a.