The purpose of this article is to prove that Gersten's conjecture for a
commutative discrete valuation ring is true. Combining the result of [GL87], we
learn that Gersten's conjecture is true if the ring is a commutative regular
local, smooth over a commutative discrete valuation ring.
In new version, we will correct several mistakes in the ornginal one.
Especially, we will give the new proof of the retraction theorem (Theorem
3.13).