polymerizing an olefin with a single site catalyst in a reactor, while controlling the amount of a halogen-containing compound that is not a catalyst component in said reactor such that the molar ratio of a halogen atom in the halogen-containing compound to a transition metal in the single site catalyst is 0.8 or less, wherein the halogen-containing compound is derived from at least one material added to said reactor and/or at least one material produced in said reactor, and said controlling comprises assaying the amount of halogen atoms present in the reactor by at least one of the following three methods:

a. measuring the amount of the halogen atoms contained in the materials in advance, and assaying the amount of the halogen atoms present in the reactor from the amount of each material introduced into the reactor,

b. assaying the amount of the halogen atoms present in the reactor after each material has been introduced into the reactor, or after manufacturing conditions have become a steady-state in the case of continuous process, by directly sampling the halogen atoms from the reactor, and

c. determining the amount of halogen atoms by sampling halogen atoms in the middle of the line circulating each material, and assaying the amount of the halogen atoms introduced into the reactor from the balance of the amount of each material circulating in the reactor and that discharged from the reactor.