To provide a polyester-cotton blend spinning material having durable resistance to washing in low cost, and provide a flame retardant processing method that can give a polyester-cotton blend spun material the durable flame retardancy resistant to washing in low cost.

After radiation with rays, radically polymerizable phosphorus-including compound is applied to prepare the polyester-cotton blend spinning material. In the flame retardant processing method for the polyester-cotton blend spinning material, the polyester-cotton blend spinning material is radiated with radioactive rays and a radically polymerizable phosphorus compound is applied to the radiated blend spinning material.