PURPOSE: To short-stop a polymerization reaction quickly and eliminate adverse effects caused by post-polymerization, by (co)polymerizing ethylene with the aid of a Ziegler type highly active coordination polymerization catalyst, and then deactivating the above catalyst with a specified compound.

CONSTITUTION: Use is made of a titanoxane compound of the formula (wherein R is 1W20C hydrocarbon group, and n ≥2), e.g., decaisopropoxytetratitanoxane. Namely, ethylene or an ethylene/1W18C α-olefin mixture is polymerized at an average temperature ≥30°C with the aid of a Ziegler type highly active coordination polymerization catalyst containing a transition metal compound and an organometallic compound, and then above titanoxane compound in an amount of 0.4W8mmol (in terms of Ti) per mmol of the total of the transition metal compound and the organometallic compound in the catalyst is added to deactivate the catalyst.