  Particle-in-cell (PIC) simulations of collisionless magnetic reconnection are
performed to study asymmetric reconnection in which an outflow is blocked by a
hard wall while leaving sufficiently large room for the outflow of the opposite
direction. This condition leads to a slow, roughly constant motion of the
diffusion region away from the wall, the so-called `X-line retreat'. The
typical retreat speed is ~0.1 times the Alfven speed. At the diffusion region,
ion flow pattern shows strong asymmetry and the ion stagnation point and the
X-line are not collocated. A surprise, however, is that the reconnection rate
remains the same unaffected by the retreat motion.
