  Quantum bit commitment has been known to be impossible by the independent
proofs of Mayers, and Lo and Chau, under the assumption that the whole quantum
states right before the unveiling phase are static to users. We here provide an
unconditionally secure non-static quantum bit commitment protocol with a
trusted third party, which is not directly involved in any communications
between users and can be limited not to get any information of commitment
without being detected by users. We also prove that our quantum bit commitment
protocol is not secure without the help of the trusted third party. The proof
is basically different from the Mayers-Lo-Chau's no-go theorem, because we do
not assume the staticity of the finally shared quantum states between users.
