We performed ring-opening polymerization (ROP) of ε-caprolactone with dilithium tetra-tert-butylzincate (TBZL) in toluene. TBZL is a bulky zincate complex with a range of unique reactivities such as unusually low basicity and high chemoselectivity; it is a metal catalyst of low toxicity. The initiating species for ROP of ε-caprolactone is, however, not the tert-butyl anion of TBZL but the hydroxyl anion derived from a trace amount of H2O in the system. Instead of a trace amount of H2O in the system, a known amount of benzyl alcohol (BnOH) was added as an ROP initiator. Poly(ε-caprolactone) with a BnO group at the initiating chain-end was obtained, as expected. 1H and 13C NMR spectra of a TBZL–BnOH mixture suggested the formation of a complex; the complex was the initiating species. The Mn increased linearly with increasing conversion. This means that the number of polymer chains is constant throughout the polymerization. The molecular weight distributions were relatively narrow, except with increased temperatures and/or prolonged polymerization times. This system does not need a high polymerization temperature or long polymerization time and uses a metal compound that has little effect on the environment. Poly(ε-caprolactone) with a narrow molecular weight distribution can therefore be obtained under mild conditions.