Motion planning in the presence of time-dependent obstacles 
is studied. Many of the prior approaches to motion planning 
problems assume that the obstacles always exist in the 
environment, whether they are stationary of in motion. In 
this article, I consider the case that the environment contains
obstacles whose existing periods are dependent on time (i.e.,
they can appear and disappear in the environment). This formulation
allows us to model a variety of situations that can arise in
application domains. A concept similar to that of visibility
is used to solve the problem. An algorithm is presented to 
generate a motion in such a dynamic domain, its time-minimality 
is proved, and computation time is analyzed.