PURPOSE: To provide a method for accurately measuring the receiving level of a received melted plastic in a raw material-mixing tank of a waste plastic-treating installation for receiving the melted plastic, and subsequently successively supplying the melted plastic into a thermal decomposition tank in the next process.

CONSTITUTION: In this raw material-mixing tank of a waste plastic-treating installation for receiving a melted plastic and subsequently successively supplying the melted plastic into a thermal decomposition tank in the next process, discharging pipes 7, 71 for the received melted waste plastic B are disposed at plural positions in the height direction of the inner surface of the tank, and a leveling meter 8 is disposed in the discharging pipe. Further, a port for discharging the high temperature liquid waste plastic and a port for discharging an inert gas for purging are disposed in the discharging pipes 7, 71. Since heating and purging means are disposed in the discharging pipes having the leveling meter 8, the inner clogging of the discharging pipes 7, 71 with the received melted waste plastic can be prevented. Thus, the level of the received melted waste plastic in the tank can accurately be detected. The installation can thereby stably be operated, because the received melted waste plastic can normally be supplied to the next process.