To provide a blow operation method for waste plastic in a melting furnace wherein upon waste plastic being blown from a twyer of the melting furnace combustion property of the waste plastic and conversion property of combustion gas are secured to solve a problem of processing the waste plastic, and utilize it as an energy source, and further improve productivity.

In a blow operation method in a melting furnace where fan air and an auxiliary fuel are blown into the melting furnace through a lance installed at a twyer of the melting furnace, upon waste plastic being blown from the lance one kind or more among polystyrene, polyvinyl chloride, and polyethylene terephthalate resin all having high thermal decomposition property is or are mixed previously by 30% by mass into the waste plastic chiefly containing polyethylene and polypropylene.