To provide a method for improving catalytic efficiency, with which influence by an extractive solvent is reduced or eliminated in producing an olefin polymer by using a liquid hydrocarbon mainly containing a 4C olefin obtained from a raffinate of an extractive distillation comprising a diene-based hydrocarbon as an extract in the presence of a granular solid acid catalyst.

A liquid raw material hydrocarbon is brought into contact with an aluminum atom-containing treatment agent and polymerized in the presence of a granular solid acid catalyst containing ≥60% by mass of particles having <1,400 μm particle diameters.