PURPOSE: To improve an efficiency of grafting, by controlling the relative amounts of an initiator and components to be fed in the continuous grafting reaction of polyolefin wax in a molten state with a styrene monomer in the presence of a radical initiator.

CONSTITUTION: A continuous reactor is continuously fed with molten polyolefin wax (e.g., polyethylene wax or polypropylene wax), 0.05W30mol per kg of polyolefin wax of a styrene monomer (e.g., styrene or vinyltoluene), and 1×10-3W 1mol per kg of polyolefin wax of a radical initiator (e.g., di-tert-butyl peroxide or dicumyl peroxide). The continuous graft reaction is effected under such a condition as to melt and flow the polyolefin wax to obtain the purpose styrene monomer-grafted polyolefin wax.